My List!!! Putting it under a cut because it has spoilers for Shattered World AND City Between, and also it is Very Long:
Okay, post in which I have compiled all the evidence that stood out to me on having been informed that one of the characters who lives in the Tea House is the illegitimate son of the former king of Behind and one of them is the other heirling whose parents chose to save him rather than themselves!
I've picked up all the seemingly relevant details and sorted them into headings- evidence for and against Jasper and Luca respectively being each of those two things, plus some extra details that seem relevant but don't quite fit into any category.
Jasper is genuinely the king's son:
default assumption of the narrative
Jasper's father "changed the landscape of Melbourne"
Jasper resembles the king in some ways: tall, also golden brown hair? Glasses + demeanor
the windows in Jasper's house/workplace just like the windows the king had in his house
the implication about Jasper's bad fae magic being an insult to his dad would fit with the king expecting his son to be powerful
Jasper's dad had a terrifying ambition for power
Jasper's dad Makes The Most Use out of humans/people in general
Jasper's dad draining people of magic... I think that was something the king did
Jasper's knowledge of Behind politics
Jasper and Jonno met in some sort of part-fae boarding school; we know the king's son "Tammy Simmons listed at one of the most prestigious boarding schools in Melbourne"
Jasper's dad is more broad and tall and has more/longer hair than he does, which does sound like it comparatively could be the king
Jasper's present tense dad is a memory, not alive
The fact that Jasper was raised by a different dad would make sense if he was illegitimate
or, if he's the legitimate son, we know Tammy's mom died young, so THAT could explain him having been adopted
Jasper is theoretically at least half fae from his father (which can lead to the assumption that he should bleed blue, based on what we know of half fae characters and what Luca has claimed about Jasper specifically)
Jasper is not the king's son:
the extent to which it is assumed and distinctly never explicitly confirmed that Jasper is the king's son is just suspicious
"You should ask your dad why your magic isn't stronger" - The king had a lot stronger magic and worse between ability, which seems to be the opposite way around from Jasper
Jasper says his evil fae dad drained people of magic for 40 years, which sounds like something the king would do but which is a much shorter length of time than would be expected of the king
Jasper and BoRa both refer to Jasper's dad in the present tense
Jasper calls his dad "not quite human," which doesn't necessarily entirely imply fae? "Not exactly alive but isn't dead either" also doesn't entirely imply the dead king.
the two powerful benefactors- you'd think the king's son wouldn't necessarily need benefactors other than literally just the king himself
Jasper is the Queensland boy:
Jasper says he was raised by non-biological parents, who both died when he was a child
finding out his father was fae after his earlier parents died + the fact that he is at least partially human on his mom's side (assuming by his own acknowledgement of his humanity) offers the real possibility for him to be an heirling
Jasper is not the Queensland boy:
Cora's story about the Queensland boy is assumed to be about Luca but never explicitly stated; she bookends the story with comments about Jasper and says maybe it's not just painful but "an inconvenient truth" to him (which seems more of a Jasper way of thinking than Luca)
I think the main thing that precludes this is the idea that Luca IS, in which case Jasper can't also be. But in the event that Cora was not referring to Luca, I have not seen any details on Jasper that would disallow him from being the Queensland boy.
Luca is the Queensland boy:
Luca says that his dad was murdered and it was messy and not a good experience
Luca says that both of his parents are dead and he must've been the one who killed them because no one else was there to do it ("Who else was there? Ask the police, they'll tell you there was no one in the house but me. Caught red-handed...")
starts to ask Viv if she has a person hidden in her house
only kills Behindkind who hurt humans- wants to defend humans from more powerful Behindkind the same way Pet does, just more violent about it
parents' murder: wakes up to darkness and heat and smell of blood, blood all over the floor and walls and in the carpet, exactly like Pet's story (while he does claim it wasn't true, he doesn't specify how much of it- and would he accidentally invent the exact same story as the other survivor?)
Luca's significant nightmares
makes a comment to Denise about the climate in Queensland
comment about Behindkind killing humans but only after they play with them first
talking about becoming Very Human or Very Other to survive being thrown into a world you're not naturally from in response to Viv saying he's just as strange as someone who's not human
very bothered by others not caring about the humans who die
the way Luca is bothered by bad parents (like the way he can't seem to understand why Viv's dad treats her like he does) gives the impression that he has an expectation that parents should treat their kids well
Cora tells Viv the story of the Queensland boy when Viv asks about Luca, seems to be looking at and referring to him
he DOES tell Viv at one point not to stand in a certain place by the end of his bed, like Pet with her nightmare, and he takes her not standing there anymore as confirmation that she's not there to kill him
"I forgot a lot of things when I left the human world," implying that he started out in the human world and then went Behind for a long time like the story (could explain the reaction to the idea of being human, like he'd almost forgotten he was human too?)
hates fae in particular the most
forgetting about the nightmare...? (book 3, between nightmare visits)
Luca comments about expecting Viv's house to have a secret room, which would make sense for a character like Pet who was never found by police after the parents' murder due to the presence of a secret room
Luca is not the Queensland boy:
"Ask the police... caught red-handed" doesn't sound like a kid who the police were never able to find (intentional clue that it WASN'T him or small detail the author forgot?)
Luca's nightmares show up in a lot of different forms but have not presented an obvious Athelas nightmare
he makes more negative comments about parents than you'd expect if he had parents who saved his life- "That's the thing about parents- you think they wouldn't do something and then you have two knives and a silver thimble in your stomach," "I'm very good at not saying things to parents," etc
says that he killed his mother even after the initial phase of saying upsetting things to bother Viv
lot of nightmares where his mom is the scary thing (though is she the monster or is it upsetting that something traumatic happened to her?)
this is the kind of misdirection that happens in these books- it's assumed and never confirmed that the comment about the king's son refers to another character, which is Suspicious.
Luca also resembles the king- golden hair, scruffy, etc
Luca's name- when the king's legitimate son was born, the name the king went by was Lucas
Luca initially tells Viv he's not human, but after she says he IS, he is Fascinated And Delighted and keeps repeating it
says his magic came from his father's side
telling Viv he wasn't going to kill her, just cut her arm off- parallel?
Jasper claims that he counts as more human than Luca, which would if true imply that Luca is half human at most
"it's rude to ask people about their age" is a fair comment BUT it does also obscure information... (doesn't say he's older than he looks but also doesn't confirm he actually IS like 30)
Viv describes Luca's haircut as looking like it's out of the '40s, which could imply that he's older than he looks
water trauma is related to childhood kidnapping- why? King's son would be a very reasonable kidnapping target
behindkind responding when he got back from being kidnapped, not humans
Luca hasn't said nearly as much about his dad as about his mom, which could imply something like a single parent situation
his mom said she had bad taste in men
Viv notes Luca's room looking like it's from the '90s, which is when she was a kid and he would've been a kid if he's her age, but then she also notes there are a lot of decades represented, going all the way back to the '50s
Luca is not the king's son:
Luca does not resemble fae in the way other half fae characters (ex Jasper, Jonno, Zero- even BoRa) do
injured on page multiple times and bleeds red like a human
tells Viv things can be identified by their blood when they have a knife to your throat; could signify narratively because of the fact he tries to kill her with a knife to her throat later that his bleeding red is a genuine indicator
also "you can't think you're human when you're bleeding like that" to the Maker- I don't think Luca would be bleeding red if he was at least half fae
says he's human, other characters generally seem to think that makes sense, and he doesn't otherwise seem to lie a lot
seems to require more sleep than other half fae characters and functions badly when he doesn't get it
neither the claim that he killed his dad or that his dad was a hero who saved his life (which he says twice) would make sense if his dad was the king
says Jasper's healing is bad because it's fae magic, which implies that Luca's healing is not fae magic
magical ancestry may or may not be Something Else
calls the nightmares the result of what happens to a human mind after being Behind for too long
says his mom was much taller than Viv- I can't tell yet how much the author accounts for genetics in her made up stories, but we know the king was tall, and unless there is a significant reason, two taller parents should not add up to a very short child!!
Other relevant confusing details:
Jasper and Luca BOTH separately say they count as more human than the other
Queensland heirling- Pet asks how come all the other heirlings but her are from the 20s, but she hasn't seen this file, but if he WAS in the group from the 20s (why wouldn't he be?) he COULD be half fae 😶
though Cora's story about the Queensland boy says he was lost Behind for about 20 years until very recently, after his parents dying when he was around 8, which would put him around human 30
book 4 Cora references Jasper's childhood as having things torn away from him in the bloodiest way possible and then surviving in a place that values death and cunning over love, which could match up with the king's son but also fits very well with her Queensland boy story (she says "you know about his childhood now" or something like that, almost like she's implying that she's already told Viv about Jasper's childhood?)
Jasper and Luca BOTH the king's sons???:
City Between brings up the king's legitimate half human son, Tammy, and then Shattered World gives us the information that the king has apparently multiple illegitimate sons- they aren't necessarily all going to be in the same place, but there are multiple sons in the picture to account for (+ why introduce multiple sons if the fact that there Are multiple is not going to be relevant?)
the chollima mentions people getting killed and houses burned down, which are both Queensland Boy Details, in reference to the king's illegitimate son, which could imply that the Queensland boy IS the illegitimate son
the fact that we are told there's one of the king's illegitimate sons here AND Jasper shares major backstory points with legit son Tammy could imply they they are one of each
the whole first meeting conversation, where Jasper says he knows Luca and Luca says they know OF each other but don't know each other, with an implication Viv doesn't get but Jasper does, and he also says "we don't know each other well enough" like they're at least vaguely familiar with some relevant identity in regards to each other...
last point is also interesting combined with Jasper talking about Luca using the phrase "has always been" like he's KNOWN Luca longer than they apparently have. Jasper is always making comments on What Luca Is Like like he knows
even Viv from the first book is thinking Jasper and Luca know each other better than they admit
Jasper and Luca DO have a lot of off page one on one conversations
the way Luca is trying to irritate Jasper from the start could just be Luca or could very easily be Annoying Brother Behavior
the way it looks like Jasper let Luca go on purpose?? Why would he WANT to take Luca out of high security prison and then intentionally let him free?
I would love to try to discuss any of this!! It's so long I don't know if everything specific can be addressed but I would love to talk about any and all of it.
Mostly I think it just makes me more certain there is intentional obstruction of details going on and something more complicated going on with all of this than what we're seeing so far without any certainty on what that is. I feel more excited to wait and find out since I can't really pick a solid answer out of it 😂 I think At The Moment I am leaning most toward thinking maybe Jasper is Tammy AND the Queensland boy and Luca's deal is Something Else Entirely