because after the timeskip, we can see that he's visibly grown taller and more mascular + there's also the fact that they quite literally censored his grown up appearance in the afterword, so it's clear that the author gave the little high school vampire kid a complete glow up after military. and also, it's kind of just an unspoken understanding in yaoi fandoms that the physically bigger guy is the top (it's a stereotype and it shouldn't be that way, but oh well)
Whats has being taller and masculinity to do with being a top
i said its a stereotype and it shouldn't be that way dawg but it unfortunately is and that's why the fandom unanimously decided that he'd be top
ohhh i seee thank u thank u!
how does everyone know the vampire is the top? not complaining. just genuinely asking