I'm with you. Killian is the character with the most depth in this entire game. A bullied, meek little kid. Grows up to take his position in the upper echelons of society. Handsome. Rich. Confident. Women, who never gave him the time of day, now flock to him. Mostly for the money. He becomes extremely jaded and cynical in relationships and women. Woman are nothing to him. He can get just about anyone he wants.
But through all this change, he never lost track of his only true relationship. That with the MC, the only one who looked out for him when he was weak. Even though MC kind of grew distant due to his education and separate social spheres they inhabit, Killian was the one who tried hard to keep in touch. When they finally have the chance to reconnect and live in the same world, he is such a genuine bro. There is a stark dichotomy in which he views other people, including his parents, and how he treasures the friendship with the MC. By extension, he treats MC's mom with respect. She was likely a mother figure to him as growing up too. We're not too sure if there is any sexual attraction, but I don't think so. She is the woman that showed affection, unlike his own parents.
Then comes Mina. This bubbly energizer bunny who exudes positivity. I think Killian feels that she is different. She isn't in it for the money. He still can't exactly change his deep rooted cynicism about love, but given time, she could definitely change him.
That's where I come in. The player. Mina is too fucking cute, so I do want to bang her at least. But I will feel way too horrible doing my bro like that. So I give myself some wiggle room. I don't ever talk shit about Killian or sabotage the relationship. But I can't help it if my hand slips a little while hugging her in a totally platonic way. Kissing her in the heat of the moment, because it was just acting practice, right? I promise... I'm not trying to be an asshole who steals his best friend's girl, but maybe some mutual feelings might develop without us even knowing! Cough... maybe we can get Killian's blessing if we are on good terms. Hell, maybe a very wholesome devil's triangle would be enough to sate my appetite. There are other girls to play with. I wouldn't mind if Mina and Killian end up being a lovely couple. After I get a taste that is.
The fact that this type of immersion and roleplaying is possible is a testament to how great the writing is. It's not too common for such complex relationship dynamics to be portrayed without much inconsistencies in character motivations and actions.
Cheers.