#tl;dr the demon is a metaphor about dissociation and trauma and it's doing its job thematically fucking pitch perfectly that way the end

[vaguely-concerned]

thinking about not only the specific people lucanis pulls in to represent the 'locks' in his psyche, but the storytelling that happens in the structure/order of them. the underlying ideas are presented something like:

the lucanis who went into the ossuary never came back out again; he died down there (the boy caterina raised is gone forever) -> you're putting yourself in danger doing this (by being close to me), you should leave because I can't bear it if you get hurt because of me -> it doesn't matter even if we do try this, it won't work anyway (again because of me) ('you know what he's like, you can open the door but he won't walk through it' :'( oofie doofie) -> what if the real secret is that there was never anything but the monster in here from the beginning. you should leave, there was never anything here worth saving in the first place. (implicitly: what if I deserved what happened, all along.)

it runs pretty cleanly from outward-oriented attachment anxiety ('caterina won't even want me back like this, she won't recognize me (the same way I no longer recognize myself)) and gradually deeper inwards until we reach self-image and self worth. or you know, the harrowing basic lack of it lol.

"careful -- they'll know we're not right," spite says in one of their first scenes... but clearly, some very deep part of lucanis has feared or suspected for much longer than that that there's something inherently not right at the core of him, way before any demon entered the picture. and the voice he gives those lines to is the person who should know him better than anyone in the world, who he has loved more than anyone in the world -- and who deliberately chose to hurt him so horrifically anyway. 'It's better if I'm just a monster and deserved what happened than it is to allow for the idea that the brother I love doesn't really exist and maybe never did'. it's better if he's fundamentally flawed in some way that needed fixing to help him survive, and that's why caterina chose to hurt him again and again -- out of love. (this one I think he might have a very sad wakeup call on one day if he ever ends up with the responsibility and care of a child of his own in some way and realizes just how alien the idea of ever intentionally hurting them for any reason is to him. oh buddy. also interesting that he keeps caterina as the outermost lock -- there IS a distance he keeps there that he hasn't with illario. he doesn't resent her 'anymore' he says, but he also keeps her carefully further away from his deepest self.)

as far as I could tell the only note in the mind prison that's fully hidden and needs to be uncovered is the sad painful helpless stupid little truth that even after all this, even knowing what happened... he still loves his brother. is there anything illario could ever do that would make lucanis completely stop loving him, do you think? sometimes the trouble with unconditional love is that it is, well. unconditional, even when some terms and conditions probably would have been in order haha.

that's the pattern you see there again and again; he would rather destroy and abandon and imprison himself at every turn than let go of love, even when it's just scraps, even when there's only ever enough of it to hurt him. it's only when rook shows up and as it were takes his hand and walks along with him that he can entertain the idea of changing the story of what walking out the door might mean in the end.