Tbh I wish we had more fan content focused on how Mika used to cover up how miserable he was with a happy facade and now he just…doesn’t have the strength or willpower to keep it up anymore, and /especially/ how Yuu knew he was hurting and tried to help in any way he could. Like, I know there’s a tendancy to focus on how Mika shifted from being a happy-go-lucky person to someone who’s cold and shut off, but I don’t think there’s really been as much focus on the fact that he was struggling /when/ he was positive? And there’s usually a lot of fan focus on Yuu being sad about Mika losing his smile but honestly…in canon I’m pretty sure that Yuu wouldn’t be as sad about Mika losing his positive attitude as much as he would be upset by the fact that Mika doesn’t have the energy to keep it up anymore. Mika’s personality shift isn’t really so much as a drastic change as much as it is him giving up. Before, he had to keep up appearences so his family wouldn’t get worried, but now that they’re gone, what’s the point? The only person in the world he cares about who remembers him like he used to already knew that it was all a lie anyways. He’s got no reason to keep it up. Plus, the vampirism isn’t exactly helping, either.
Idk, I just think that it’s an aspect of his character that isn’t really thought of enough