A few ideas that came to mind for the Vessel!Knack concept. The characters most likely to get affected are those who possible bad endings, died or been in a situation which had separate their soul from the body. Now I'm going to do something a bit differently around Knack 2.
The second player or Blue Knack is a prototype vessel for the other soul in our hero's chest orb. Heavy emphasis on prototype as the moment it breaks then the other spirit returns to their previous containment and the destroyed body needs to be repaired. There are further changes than just this.
Unlike in Knack 2, this temporary vessel will mimic physical traits of the soul when using relics to increase their side. Their Sunstone powered abilities also change to fit the respective fighting style. When combining with Knack like in the game's Co-Op, the appearance and abilities are much closer to an actual fusion.
There is one more detail I sorta forgot to mention. Should Knack be in a area heavily saturated in Relic or Sunstone Energy than the spirit within his chest becomes visible to everyone around him. This 'form' acts more like a phantom as their voice can be heard but they can't interact with the outside world.
Now with that outta the way, we can truly get started.
Jonathan Joestar
Figured I start with a pick from an older fandom I'm in: JoJo's Bizarre Adventure. Couldn't resist the irony of an archaeology enthusiast awakening inside sentient ancient relics. Jonathan isn't fully aware about this new situation at the start.
His mind's really hazy during Chapter 1 of the first game. Jonathan barely get a few snippets from Knack's viewpoint so he's heavily confused about this new situation. Unlike the other picks, he becomes conscious much faster due his and our golem hero's powers being connected to sunlight.
Not exactly a good thing as frequent vision obscuring illusions(memories) throughout very hostile uncharted territory full of bottomless pits is an awful combination. Or that it makes your allies concerned about unknown glitches being left unnoticed until now. Doctor can't exactly reboot Knack.
The first swap between both souls occurs during the boss fight against the Goblin Chief. For those unaware, there is a unique martial arts in JJBA called Hamon. A breathing technique which allows one to channel sunlight like energy for various effects from healing to enhancing one's own weapons. Combine that with a golem who can channel sunlight into different forms of destructive attacks and you got a lethal force to be reckoned with.
Knack is essentially the perfect vessel for Jonathan to take his Hamon to unimaginable heights. The golem however was a bit rattled upon seeing the Goblin Chief's mech utterly destroyed in a blink of an eye(to him). A black out that continues to happen even more frequently.
Doctor and Lucas being the only ones to help piece together this strange new personality for Knack to understand. Though the story Jonathan tells is very bizarre to them with his evidence being the Hamon technique. A stone mask that turns those who wear it into an eldritch vampire and a evil foster sibling that uses said mask for world domination.
Now neither souls has seen their shared body so Jonathan is a bit suspicious about everything that has happened so far. (He thinks he is in a trap made by Dio.) At least until the golem gets tossed into the ravine and sees the spirit. Jonathan's phantom like form visibly apart from Knack's own destroys the remaining doubt.
It takes until final boss of the first game for both to become in sync. Jonathan still has difficulty trying to adjust to this new reality. You have to remember that his psychotic vampiric foster brother is loose in his world and completely unaware of what he's up to.
Knack does try to ease Jonathan's worry, usually letting the man take control so he could take a walk or blow some steam. The Joestar is quite happy once the Doctor creates his vessel as he won't lie that being cooped up without a body tends to suck. He does miss his old one but makes it work in the end.
At least Jonathan can officially try to become an archaeologist! He can't exactly stay far away from Knack but studying relics goes hand in hand with his work. A win-win situation although Jonathan might have a few bumps in the road.
Those being Knack 2 but also a destined encounter involving his past and the bloodline he left behind...
Sparda
I figured the next pick would be a character who I haven't experimented with that much. Plus someone needs to kick this devil's ass into gear at some point. Why not shove the Dark Knight's soul inside a the core of a little size shifting golem.
It takes longer for Sparda to gain consciousness since devil spirits take a bit longer to fully connect to a non human vessel. This has led to his power seeping out on occasion. Knack often finds his attacks or abilities more destructive than normal. Only warning anyone has is the chest orb's crystal flashing violet.
First swap happens at the very start of Chapter 4 once Knack connects with the giant relic in Viktor's mansion. Sparda is super confused and concerned about this new situation. He is unaware of what to his wife nor kids.
This body has another spirit inhabiting it whose already on their own respective mission. Sparda helps the group nonetheless as it may lead to more answers. Knack's a bit conflicted about him considering most of the memories he seen from the devil are quite gruesome. (Devil May Cry games in general very violent plus it took some time before Sparda decided to fight for humanity.)
It took quite awhile for Knack to become accustomed to the more brutal memories but does decide to trust Sparda in the end. He feels less instinctively isolated with the devil around from how different they are to others. A feeling that allows them to fully become in sync around Chapter 10.
Once the temporary vessel is made, Sparda's routine sums to wandering, reading or training. It is the only thing to keep him from overthinking about the family he left behind by his death. Knack does try to help Sparda with his worries even if the most the golem can do is reassurance.
The devil does drag Knack into some of his work routines which includes swordsmanship. He figured it would do some good that the young hero expands his skill set as he really doesn't have a fighting technique at all. Sparda stills retain some of his magic and often contemplates about making new Devil Arms from it.
He often tests the Doctor's new inventions mainly out of curiosity. A great way to spend some time but also experiment with the possibilities. Sparda wonders if he can incorporate Sunstones into magic or creating Devil Arms.
A thought that might decide whether the two will be ready for their new adventure. Or when the time comes for Sparda's past comes to show itself. Guess it's time to pull that Devil Trigger.
Tangeth Toborn
I decided the final example should come from a series that has faded into the background for a decade, Chaotic. The heroic lion-like Overworlder who gave up his life to save the world of Perrim: Tangeth Toborn. Lore wise, he came from a place called the Spiritlands and could only exist outside the realm through his weapon, the Sword of Kh'yat.
A blade later broken to summon him when a dangerous threat from said land came to attack Perrim. Taking this tidbit into consideration, he's the second fastest to gain consciousness as his spirit still had issues adjusting to Knack's chest orb. It did speed up the bombardment of visions between Tangeth and his new host.
Knack had to take a short break multiple times as the memories were so vivid to the point he gotten minor dysphoria. Lingering taste of foreign food, the stickiness on matted bloody fur, and hot flames that burn his flesh. Tangeth wasn't having fun either as it became apparent he's in the human world. Or at least some variation of it. (First Creature on Earth!)
First switch during the boss against the Goblin Chief like in the Jonathan scenario. Tangeth uses fire and earth elemental attacks which took some time to learn how to use again since Knack wields solar based energy. He also wanted to know the entire situation before he willingly decides to help the Doctor and Lucas.
Tangeth truly begins to open up during his shared time with Knack down in the ravine. Chaotic Players(humans) have often caused chaos for his tribe to the point that some were willing to destroy precious locations for personal gain. Knack is the only person who's able to get Tangeth to at least give this new world a chance.
Both fully synchronize with each other halfway through the last chapter of the first game. Tangeth adjusts very quickly to the prototype vessel the Doctor creates much faster than expected. He does have experience with soul binding artifacts after all.
Tangeth usually spends his time reading, exploring or training. He has some difficulty trying to adjust in a world with way fewer conflicts between its nations unlike back in Perrim. Tangeth later asks the Doctor to make him a new sword, one powered by Sunstone he calls Kiru. He swore a new oath on the blade that he protect this world from danger.
Tangeth has a tendency to drag Knack into his training sessions as he also believes the golem should learn some actual fighting styles. Why waste such potential with just simple punches and kicks? Yes, Knack ate dirt a few times cause Tangeth was a general for an army before his death so no holding back.
It definitely helps when the next adventure proves itself to be much harder than the last. Or the possibility of a certain dream episode could potentially come true in this reality. No worse way to test loyalties.
At least that's how I see all these particular scenarios.
My trauma loving ass thought of an idea. Knack but he is a walking soul container. His spirit and another(s) in the same body unaware of each other at first. Knack often having strange dreams about being in someone's shoes thrown into various situations that range from insane to terrifying.
The golem fully unaware that these are someone else's memories who slowly becomes more conscious the deeper he goes. A change which gradually gets noticed by his companions as Knack and this other spirit begin to sporadically switch control. One second he's normal self but then the next acts like a completely different person.
To Knack, the swap is equivalent to a blink. One second he is chatting and the next atop a skyscraper like a gargoyle. He absolutely grows nervous about these blackouts as no one knows what's going on. Then Knack looks into a mirror.
He isn't alone in the reflection. Someone is standing there next to him. An impossibility since he's by himself. Knack moves his hand and this second person copies him at first before acting of their own accord.
There will soon come a time where both souls can move together as one. Able to interact without any mental barrier keeping them separated. Sadly not now and won't be for quite awhile.
Knack is stuck having to understand this phantom that follows beside him. The same goes for his companion. Both face the unknown alone and are scared of what it all means...
