girl as tricked by the new boyfriend, has some illusion magic and made her think the mc cheated on her. mc in turn doesn't explain himself even after the boyfriend explains to the mc how he used his magic to trick the girl. the mc is constantly insulted or w/e over this throughout the story and he takes it and continues with his training and adventures with his new friends. and every time the girl and the mc meet we will think "its time to reveal everything" but no we wont get that.
it's probably just me but every time i try to read manga like this with a stoic character who is "hiding" his abilities to not stand out makes me cringe a little, i feel like the targeted audience for manga with this troupe are people who can't stand out and have no ability/skills to speak of, so when they read this it makes them feel like the problem doesn't lie with them but in everybody else in the world that fail to recognize their greatness.
that's kinda the point. humans in general don't have special skills naturally so we turn to fiction and fantasy to make our boring lives more interesting.
also what you described (MC personality aside) is basically the plot of any story with a main character that has special abilities so if you have an issue with it you might want to find a different genre to read.
i feel like this girl leads men on, the mc "protects" the girl ends up killing someone and probably does some other stuff that keep in going down the crazy rabbit hole changing him until he ends up as the new "stalker" when the girl eventually starts distancing herself and finding a new white knight the save her.
That was exactly what I was thinking