Not a prompt, but do you think Lucanis has an 'oh, shi–' moment when he realizes he's caught feelings for Rook? Or do you suspect it is more of a "Won't Say I'm In Love" situation where he just refuses to acknowledge it for the longest time?
A little of column A, a little of column B.
I'm a Demisexual-Lucanis Truther, so he might not suss out his feelings for Rook right away. You know, there's that moment of confusion where you question if your feelings are platonic or romantic.
He definitely knows what a crush feels like because he had feelings for Viago in the past, but I think his "Oh Shit" moment happened during that first kiss scene in Veilguard. I mean, we actively see it happen on screen! He gets caught up in his attraction, he flirts back, he gets pulled into Rook's orbit, and then he realizes how far he's fallen right before their lips touch.
At that point, he can't deny that he has feelings for Rook. He also has confirmation that Rook likes him back, which, depending on how you headcanon it, could be a whole new territory for him.
But while he can't deny that Rook turns his world upside down, he can insist a relationship is a bad idea. That's where column B comes in, and I wish we had more of this tension in-game. We've heard it before, right? "I'm an abomination, and it could be dangerous to be with me!"
Not that we, or our Rooks, care. But there are other factors he can throw in there, too. He's an Antivan Crow, he's the future First Talon, it's unlikely they're going to survive facing the Evanuris, he's not experienced when it comes to relationships, etc.
In the end, though, his feelings for Rook are too potent to ignore. For one, he has a spirit in his head who probably doesn't leave him alone about his affections, and for two, he constantly has to watch Rook risk their lives, which is the perfect recipe for love confessions. Like, I need to tell you how much I love you before we perish, actually.
