Why does Mike repeatedly call Will a sorcerer and say his powers are innate but Will keeps saying he's not really a sorcerer and consistently clarifies he's just siphoning powers (probably more like a warlock)?
Mike knows the difference between a sorcerer, a wizard, a warlock, and a cleric. He is firm in his conviction that Will's powers are innate and we should believe him.
Sorcerer
In D&D, the sorcerer class did not actually exist back in 1987. It was introduced in the 3rd edition of the Player's Handbook, released in 2000, and I'll reference the 5th edition for the sake of an aspect to sorcerers I believe works best for Will: sorcerous origin.
There are many sorcerous origins but 5e started with two: Draconic Origin and Wild Magic. Later, Storm Sorcery, Divine Soul, Shadow Magic, Aberrant Mind, and Clockwork Soul were added, with the latter two joining in 2020. Evidently, the one that sounds the most like what Will is going through is Shadow Magic—where the sorcerer's innate magic comes directly from Shadowfell, a different and darker plane of existence. This can occur from lineage/birthline, or from being exposed to the energy itself and being transformed by it.
Makes sense for Will, right? Let's clarify something a bit more.
D&D's planes exist in what's called a Great Wheel. This wheel has existed since the first edition of the Player's Handbook. In the most updated version of the Great Wheel, there are sixteen outer planes in which the good-aligned planes are at the top of the wheel, the evil-aligned at the bottom, lawful to the left, chaotic to the right. In the center rests the inner planes (elemental matter, ie earth, water, air, fire, energy) and material planes (balance of the philosophical forces in the outer planes and the physical forces in the inner planes, ie the actual gameplay world).
Shadowfell is a material plane coexisting with the standard world. It's seen as a mirror of the darkness within the world. Sounds a lot like Dimension X, which is referred to in show by Dustin as the Abyss. As he explained in show, the Abyss is from D&D. It's one of the lower planes on the Great Wheel and is considered the plane of infinite layers. It's chaotic evil on the Great Wheel, it's got unpredictable horrors and evil, and there's a lot of monsters here. Including: demogorgons.
Now, the assumption in show is that Vecna is creating rifts from the Abyss into the Upside Down, the bridge from the Abyss to the real world, in hopes to collide the worlds together. Great assumption, except we really do not know why Vecna is doing this. Why does he want to destroy the world? Why has he been sending demogorgons and monsters across the bridge into the real world? What does he gain from all of this?
And frankly, Vecna gains nothing. This is not his plan. Back in ST4, the theory was posited that Vecna is under control of the Mind Flayer rather than the other way around. We saw the Mind Flayer enter Henry when he was banished to Dimension X by El in 1979. He believed himself to control the Mind Flayer, but he has been Flayed. Let's review the Mind Flayer in D&D real quick.
The Mind Flayer
If you forgot, the Mind Flayer is a D&D entity just like demogorgons, the Abyss, and Vecna. The Mind Flayer is an illithid (and if you played BG3, you probably know a bit about illithids and their role in the multiverse). It has psionic powers and thralls intelligent creatures across the multiverse as they believe themselves the dominant and main species. Mind Flayer's live in the Underdark, which is a bunch of underground tunnels and caves under the surface of the main world. It has has a Shadowfell counterpart named the Shadowdark. (Again, if you played BG3, this is the world—Forgotten Realms—the campaign is based in.)
Mind Flayers control the minds of other creatures, in particular commanders who will do its bidding. They are often connected within hive mind colonies with an elder brain. They feast on the brain of humanoids and consume its personality, fears, and memories.
We saw this best in S2 when Will's personality changed whenever he accessed the hive mind / Mind Flayer and in S4 when Vecna cursed the teens by preying on their fears and memories.
So why is colliding the worlds important? It gives the Mind Flayer humanoids to consume. It gives them another world to control, a larger part of their hivemind. This Mind Flayer may very well be the elder brain. (And A Wrinkle in Time gives us a hint to this, with IT, the controller of Camazotz, being a giant brain.)
Let's talk about what the elder brain can do.
Elder Brain
An elder brain is composed of long-dead mind flayers. It is the core of mind flayer communities and it can use its telepathy to initiate and maintain telepathic links with ten creatures at a time (links held up by keeping close distance). It can plane shift, dominate monsters, sense thoughts, and consume the thoughts and memories of those psychically linked to it. When the elder brain infiltrates a being, it deceives its senses and implants subconscious suggestions and influences to compel creatures toward actions that benefit its grand plan. It can even evoke the persona of any being it absorbs.
Elder brains live within a lair, which we saw when Holly woke up in the Abyss. Vecna is stationed in the center where something like a beating heart pumps above him. Oddly enough, the elder brain requires fluids and water to survive. In ST, we see the monsters of Abyss avoid water. The Abyss itself and the Upside Down are water-free. But our world isn't.
Collide the real world with the Abyss, and it now has water for the elder brain to thrive. A gate opened in Lover's Lake after all.
Why avoid the water in S2 when figuring out the tunnel system Will had been subconsciously created to make in his dreams? If everything about the Upside Down is proven wrong—as in it's not an alternate dimension itself but a bridge to an alternate dimension—then perhaps the "fear" of water isn't fully a fear of the water but a need for it. To not use it all up but in fact, begin the plan to consume the water within this world and make it part of the Abyss. To fuel the elder brain and it's hivemind.
Now a quick tangent about Vecna and his powers as an overview for something important.
Vecna
If you're reading theories, you've probably heard about Vecna and his eye and hand. This show is not going to be physically permanently altering the characters so actually losing an eye or hand is out of the question. That said, the powers Vecna's eye and hand have can still be used metaphorically and not in a direct literal sense.
A review of Vecna's eye and hand: these artifacts are grafted onto the user in order for them to gain access to power spells with little cost but their own sanity and good/evil alignment. The eye grants the user truesight, clairvoyance, dominate monster, and there's a chance Vecna can tear the user's soul from their body and control them. Flay them. We see this occur with Will and all his eye imagery. The hand increases your strength, grants access to sleep, slow, teleport, or Finger of Death—an instant death spell. Both the hand and eye together? The user is incredibly powerful: they are immune to poison, experience premonitions, have regenerative properties, and gain access to a powerful spell called Wish.
What is Wish? Probably one of the most powerful spells in D&D. Wish always you to do anything within the bounds of the campaign. Literally anything you can think of with no (immediate) charge. This allows summoning of any object, immediate regeneration for a multitude of creatures, even undo a single recent event by forcing a reroll or unmaking a weapon. Basically, turning back time. It is extremely powerful and consequently invokes a lot of stress on the caster. This stress can make it so the character is incapable of casting Wish ever again.
Destroying the eye and hand requires the Sword of Kas killing the creature that has grafted the eye and hand onto its body. Think...maybe the Mind Flayer. It has access to Vecna's eye and hand. This sword is physical in D&D but can be metaphorical in the world of Stranger Things. All it needs to be is something that Vecna does not have, something that a so called traitor has, enough to kill.
Something like...love maybe?
Now that's a lot of information. Let's go over this timeline real quick before we get back to Will.
Stranger Things Timeline
The Abyss hosts all these terrible creatures and an elder brain. The elder brain is stuck in a water-less waste of a world and has previously consumed humans that have accidentally entered it. Recall that in 1943, a ship had accidentally entered the Abyss and everyone in it was killed. Except for Captain Brenner. Yes, Martin Brenner's, or Papa, own father. Captain Brenner returned to our world vegetative but with a new blood type. He was infected with something. Brenner grows up and tries to replicate the experiment his dad was essentially killed in, out in Nevada. In 1953, one of the scientists ends up being a spy and tries to escape with valuable technology, coming to a head with Henry Creel in an underground cave. We see this memory in ST5; the spy is killed by Henry, who opens up the suitcase. While the show pulls us away from seeing what is in the suitcase, we can assume this is when Henry was Flayed. According to The First Shadow, he was briefly transported to the Abyss and Flayed. His personality completely changed after this incident and he blocked this memory from his own mind and is afraid to reenter it. A newly changed Henry began terrorizing the neighborhood by accordance of the Mind Flayer as we saw in ST4, and this frightened his mom, who reached out to Brenner in 1959. Henry ended up killing his mom and sister, the blame falling on his father. Brenner believed Henry to be the murderer and got him involved in the MKUltra project as his number 001.
The rest we have seen played out in show. Brenner discovered Creel's blood was different, infected pregnant women with Creel's blood in "experiments" to create kids with powers, El ended up the closest in power to Henry, and actually is exactly what Brenner is looking for: a way to open a gate to the Abyss. To discover what happened to his father and Henry and where these powers came from. El banished Creel back to the Abyss in 1979 and Brenner spent the next 4 years attempting to reopen that gate via the MKUltra project and abusing El's psionic mindscape powers. El eventually connected to the Abyss hivemind, creating a wormhole (the Upside Down, which we could call Shadowfell), and one of the creatures passed through the bridge into the lab. El escaped in the aftermath and Will Byers was kidnapped.
The big question is why Will Byers? Why was Will captured by Vecna / the Mind Flayer?
Why Will Byers?
The Mind Flayer/elder brain has absorbed Henry Creel. Henry was already proven to work when it came to Flaying humans and folding them into the hivemind. Will shares the same birthday, March 22nd. The year Henry returned to the Abyss, 1979, puts Will at age 8, the same age Henry was when he first encountered the Abyss. 8 years later, it's 1987. The kids who are chosen are within the 8-9 year age range. Coincidence? I think not.
Henry had a very similar personality to Will before he was Flayed. Soft spoken, awkward, struggling to fit in. He loved a girl, Patty, and tried to make things work but his powers frightened his mother so deeply that when he discovered she wanted to send him back to Brenner, this broke him deep enough that not even his girlfriend's love could save him.
In being Flayed, the original Henry Creel is forgotten. The discrepancy between what Vecna shares about his powers and the play accounts is there for a reason.
In being Flayed, one becomes forgotten as they are assimilated into the Mind Flayer, the elder brain. They as a person no longer exists.
In 1983, the Mind Flayer, through Vecna and the demogorgon, traveled through the bridge into hopes to find another vessel. Another being to consume into the hivemind. Children are already proven to work and a child mirroring Henry Creel would work best. Broken family, outcast, seemingly unloved, easier to break and use to fuel the elder brain's hive mind.
(We see with Derek and Holly at least that their families are not picture perfect, either. Their innocence and love for their families despite this makes them easy prey.)
Unfortunately for the Mind Flayer, Will is loved by his mother, his brother, and his friends. Unlike Henry Creel, these people do not fear his powers. He is loved for them, even when he is mind-controlled to use these powers for evil.
This is why Will is a sorcerer and Vecna is a wizard. Vecna, though changed and shaped by the Abyss, is drawing on the magic within the cosmos, the weave, the Abyss. Vecna is focused on ritual spell casting, as seen with his curses in ST4 to create the gates, and needing 12 kids--in particular, their minds--to begin casting his next spell for opening up more fissures.
Will does not partake in this ritual. His powers are bolstered by his love, something that is innate to him. Something Vecna/Henry does not have. Love is a charisma skill, it is instinctual. It is not calculated. Sorcerers also mesh well with many other classes in D&D meta wise, particularly with paladin. In fact, sorcadin as it's called, is one of the most versatile multiclasses in D&D period.
So...why does Will keep saying he's not a sorcerer but Mike insists he is?
POV Matters
Will's powers are two different things in their minds. Will sees his powers as siphoning, thus not really making him a sorcerer, because he believes he is only taking control of Vecna's powers and using them as a source for himself. This aligns more with a warlock, who is pact pound to a patron (in his mind, the Mind Flayer) and "borrows" their magic. Vecna's powers as we know are fueled by anger, hatred, and fear. Theoretically, this is what Will would be "borrowing" for using his magic.
Anger, hatred, and fear are not what Mike sees in Will. He sees honesty, love, and bravery. He sees the goodness in Will, and that is innate. That is Will's power. That makes him a sorcerer.
Even then, with Will the Wise being a wizard, Mike stating Will is a sorcerer, and Will thinking himself more of a warlock, the idea is that Will is simply magical. And with love and honesty being the answer to all against the threat posted to the party, it will be something everyone needs to channel and access to defeat not just Vecna but the Mind Flayer / elder brain as well.
Mike the Brave
You can't talk about Will without talking about Mike, so here's some discussion about Mike the Paladin and Mike the dungeon master.
Mike the Brave is a cover story. It is an identity Mike hides behind when he is scared, as confessed to Holly in 5x01. Mike the Brave explore dungeons, fights monsters, and fights sorcerers. He is brave in the face of danger and steps forward to save his friends and allies. This is who Mike wants to be, who he wishes he was, as he believes he is otherwise. In parallel with Holly being told she is Holly the Heroic by Max, we must see Mike realize he is Mike the Brave.
Bravery does not come easy for Mike. He has been hiding from himself for many years now and it is in the aftermath of Will's bravery that he himself becomes the bravest, becomes the heart. In ST2, Will's bravery in saving himself from the Mind Flayer brings Mike forward. In ST3, Will's honesty about the Mind Flayer brings Mike forward. In ST4 and ST5, the same pattern occurs. When Will steps forward, so does Mike. It gives him the space to be his best self in the wake of Will's light.
Mike is a writer as the dungeon master. He is the puppet master and controls not just the narrative but the monsters within the story. Both the campaign and his own life. Mike is extremely repressed and traumatized; he is hiding every feeling in his heart in his fear of the biggest one of them all. He is letting the monsters win and this stunts his bravery in all aspects. He is not letting himself be Mike the Brave. In fact, he simply does not believe he can be Mike the Brave. It is a character he depicts at his side, not as himself.
So who is Mike the Brave? This is Mike's paladin. Paladins are bound to an oath, which dictates the types of holy (or unholy if an oathbreaker) spells one has access to use. The oath of devotion matches Mike's beliefs the best. A devotion bound paladin lives by the tenets of honesty, courage, compassion, honor, and duty. One particular spell that encompasses Mike is the zone of truth, found in Mike's repeated "casting" of the spell with, "friends don't lie". There is also protection from evil and good, beacon of hope, holy nimbus, and others but the point is this: oath of devotion paladins are often the ideal knight in shining armor. They are about protection and honesty.
In order for Mike to become Mike the Brave, he must allow honesty from himself. The show tells us Mike is hiding something, when he is still in the room when Kali points out that Hopper is hiding his truth as well. When he is dressed in camouflage during Will's coming out scene. When he does not speak up and simply hides in the background. He is kept to himself throughout V2, barely reacts to anything, and seems to be consistently thinking. About what, we are not told, not yet at least. Whether Mike is Vecna'd / Flayed is up to the finale to decide, but in the interim, we can say he is definitely dealing with something internal.
Mike was loud and present in V1, openly touching Will, helping bolster Will's plans, being a proud and firm leader. He hugs Will and spends most of the beginning of 5x05 praising Will for being a sorcerer. But the moment Will is tranced, the moment Vecna attacks Will's mind and shows him his greatest fears, Mike steps back. In fact, if you watch Mike and Lucas rush to Will after he collapses into his trance in 5x05, Mike reaches out a hand to touch Will and then immediately pulls back. He remains at the radio station and keeps an eye on Will, but doesn't do anything but watch. No touch, no words, just a worried gaze. He then argues with El and keeps telling her they'll all leave Hawkins, but it begins with getting Will back.
Will is Mike's heart. He cannot function without it but he is scared of losing it. He told Will this in ST4 and during Will's confession, he is visibly attuned to the words of pushing people away and ending up alone. It is his fear, too. This fear of being alone. This fear of being different. But just as always, Will takes that brave first step forward in the face of fear, and now that gives Mike options he did not know he had before.
Almost like a Wish come true, the undoing of an event: of a premature self-rejection which now allows for a confession of his own.
Either way, love and devotion, honesty and courage, these things matter. It is the way to defeat Vecna and the Mind Flayer. It is the way to defeat the elder brain before it flays the entirety of the world.
Now, back to that sorcadin comment from earlier. One of the best multiclasses available in D&D, sorcadin is a powerful charisma-based combination of potent offensive spells from the sorcerer class and healing and defense from the paladin class. Will the sword, Mike the shield.
(Also note that Mike is often blocked to the left of Will, near consistently, even when they're fighting. Left hand holds the shield, right hand holds the weapon.)
"M" marks the spot. Mike is the heart. Will is the bridge. There's an X over Will's heart when he's looking at the wormhole drawing. Mike and Will are circled together in 4x09 when El is describing her piggyback plan. They are a pair and they will be able to defeat the Mind Flayer / elder brain together as long as there are no more secrets, no more fear. Only love, bravery, and honesty.
Mike must face his fears and trauma, he must accept he is Mike the Brave and always has been. He is gay and in love with Will and he can tell him and know he will not be pushed away for it. Will's Vecna vision will not come true because Mike loves Will back.
Mike can love and be loved in return. Will can love and be loved in return. And their love is more powerful than the Mind Flayer or Vecna can ever imagine.