MC a spineless wet nudle - typical "harem" MC.
Ex-GF - tried to pull the "make him jealous" BS like the CHF in "The Girls Who Traumatized Me Keeps Glancing at Me, but Alas, It’s Too Late". The other one wasn't in a relationship and actually did date another dude just to spite MC, this one was dating MC and lied about cheating. Both equally retarded if you ask me. This one thou spent like multiple months of no-contact, not trying to "solve the created issue".
Maid - seems to be full blown manipulative toxic bitch. Used MC personality to make him meet her, got angry at him accepting dating her out of pity - which means she has enough brains to understand/see that thats what it was and stuff.
Kouhai - out of them 3, she seems as best girl for now. Sure her words hurt him and bullied him, but it mostly was your typical "shy, cant be true to herself, childish, tsundereish" behaviour. She fights with herself to be true to herself, and thus to MC, confessed her feelings to MC and genuinely, without any manipulations/stuff asked him out on dates.
So like for now, MC should fix himself, and then date Kouhai or a new "non fucked-up" girl. Maid/Ex as long as they have equally much to fix in themselves as Kouhai-chan, the things they need to fix/apologize for are in my eyes far worse.