just a temporary idea for magma, but I think it's cool (for me, not pale) that humanintruder is Protag's best friend long before( and he's one-sided) and it’s big enough for intruder always jokes that "if we're __ and not married yet, we'll get married." to Protag.
but on Protag's side, he just thinks that humanintruder is just his best friend and nothing more, and really doesn't like that kind of overly friendly joke of his (plus points if he's homophobic).
And because humanintruder's clinginess is so much with him ( he tolerating him), when Protag goes on a date, he hides it carefully from intruder for fear that he will become a third wheel, and could destroy his relationship with Vera (he just thinks that because of intruder's clingy personality, he still loves him AS FRIEND). so it wasn't until Protag became engaged to Vera and the wedding was near that he told intruder.
Of course, that broke intruder's heart, his love, his only crush and obsession suddenly belonged to someone else while the person he loved didn't know about his feelings. that broke intruder so much that he cried, begged or anything to ask Protag said it was just a joke, wonder if the wedding stops
But the proposal was already decided and the feelings did not change at all, so after all, the intruder's begging was just a cry of pity and a wish for the remaining closeness of the protag.
Same as time, on this one-sided and broken misery of intruder, Protag only thought that his best friend was doing too much, too drama and actually gradually felt disgusted by his friend's request (like "I know you don't like change, but please don’t hug me like I broke up with you like some of girlfriend")
Anyway, after that intruder gradually hid from protag's views, gradually losing connection with him. while in the dark, after several more break down in his room, and severe depression, intruder gradually became obsessively stalking protag and his life after him. goes on until the apocalypse comes ofc