Hot take, Emma is the antagonist. Marcus at least offers to share his bitches with you. He's a simpleton good at sex.
Emma pretends to be your girlfriend yet fucks and sucks everyone behind your back while in a committed relationship. Her characterization is that she's smart, so she knows what she's doing to your relationship.
Marcus never hides his true nature. Emma does.
No Offence and I don't intend to argue this point again.
I don't doubt for a second in your game.
You may well be right and she may infact be a cheater however Marcus is still an antagonist his willingness to share doesn't actually change that.
But you see in the Alpha route atleast Emma only cheats on the MC as I'm sure anyone thats played the Alpha path will tell you IF.
And only If you forget to manually set her as MC's GF.
And you set her to that after you go on a date and fuck her for the first time.
untill then she's technically not even cheating.
But since the game forgets to set it for you if you don't or forget to set her to MC's GF yourself she will cheat.
However once set to GF if your on the Alpha path (despite the fact that personally I can't stand the bitch) She doesn't cheat.
Marcus on the other hand as far as I can tell especially on the Alpha route and despite having + relation points with him.
He is it apears still trying to get in the MC's Mom and Sister's pants.
And that seems to be even despite if you had told him "look but dont touch" and to to paraphrase "leave them alone".
Now correct me if I'm wrong but that seems a hell of a lot like the actions of an antaganist to me?.