Unpopular opinion: I want their first kiss to be in their school uniforms.
The idea of the visuals of Aya and Mitsuki's casual fits? Incredible. Want.
HOWEVER
Mitsuki knows Aya crushed on Onii-san. She now knows they're "the same" in Aya's mind.... but she doesn't know that the crush itself is intact and for the last while has frequently asked if it was "okay" to be included, thought of, friends with someone "like" her while they were at school.
It's where she lacks the most confidence and feels the most burdensome. THAT'S why I want Aya to be the one to kiss Mitsuki, and why I want the first kiss to be in their uniforms. No black nail polish, no contacts, no rings, no earrings, no jewelry, no messy bun, no tattoo showing.... And not after the "prince" (school version of Onii-san's" unconscious rizz...) comes out.
Just the sweaty and uncertain Koga Mitsuki expressing trepidation or doubt, and Oosawa Aya seeing it as adorable rather than something to blow pass or dismiss...