man, how are there still some ppl who want the MC to get with koto? like wat? -_-
koto has actual issues and mental health problems: she needs real help and care from a therapist, not a relationship or sex <--- that would be the worst thing possible for her.
Koto's actions in going for an older man show her desperation for a father figure in her life, cause of how her mom treats her. She's not actually looking for love or sex, she's actually trying to reach out for someone to care, as she is looking for someone to fill that void. MC -> older man -> father and he showed her some care in the beginning of the manga, so she latched onto that right away
and as she still is a kid, it's a reality that she might also think that relationship/sex with a older man is the only way to protect herself and get attention/care... when it actually won't cause it wouldn't deal with her underlying issues and mental health problems <-- this is what needs she needs help with at her age
it's a real thing that there are women, who had personal issues with their father growing up, who go looking for males similar to their father in age/personality/looks/so on, to try and fill that void: the subconscious mind may crave a ‘father figure’ who’s able to protect, adore and provide you with the affection you missed in childhood. Similar thing for some younger men looking into older women in relationships
now back to the manga: there is no happy ending here, but if we were to imagine a happy an ending, it'd be: MC adopts Koto and moves to countryside with his daughter, all three using that to start anew. Dad/daughter will take the time to reconnect and deal with their lingering grief/emotions on the mother's death while Koto can use the move to start fresh and be with people who care for her and seek help with a proper therapist
and no, no fcking romance