im kinda 5050 on them bc mui is uh nonchalant and unbothered, it's like he doesnt prefer anything. it doesnt matter who says i love you first. mainly also because he assumes the other person understand his feelings already. he expresses his love *definitely* through actions.
i picked genya because okay first, for me, genya is yk hard headed but that bc he has to toughen himself since he was a kid. deep down, hes very soft hearted and emotional. and he gets embarrassed easily so he's the type to wait for the confession. but also at the same time, he doesn't want to miss his chance to get with that specific person (mui, in this case) so when it gets to a point of time when it's been too long since he has realized his feelings and/or he feels that they've gotten close, he will call the other person out, fiddle with his fingers (clearly nervous but he inhales and exhales and then suddenly he gets a burst of confidence), and eventually confesses.
heres a scene (dialogues) about that:
muichiro tilts his head, confused why genya dragged him outside his estate.
"i.. *sighs* i really like you, maybe even love! well idk when it started but i really really admired you bc you're a hashira at such a young age and i guess it turned into romantic admiration and well"
he rambles on for so long that mui got bored of it and approached him. genya didnt even notice, not until he felt the other's lips against his.
he was surprised for a second before he kissed back. when they separated, mui says :
"what are you talking about, genya? aren't we already together..?"
"well, i think maybe a few weeks ago. we even held hands.." mui said as he suddenly clings to him. "whatever, im tired. carry me and lets cuddle or something"
genya carries mui back in his estate and laid him on his bed, and well, laid next to him.
'when the hell did that happen..? i wouldve remember if we held hands and got together...?' genya thought to himself.
of course, he didn't know. it was when he was asleep, afterall.
