That settles it. Aggressively flirty kouhai is worst girl. She is just a fucking nuisance. She can't even do her job right and wants the protag around because she is too lazy to do anything without being pushed from behind constantly. Plus she gets all up in his face at the mere suggestion he might have a girlfriend, as if she had some kind of claim on him already.
Plus she has ugly hair and worst body. She is losing on 100% of fronts.
Protag should probably just try harder with the boss, but odds are he'll end up romantically involved with the JK.
Better than the idiotic kouhai at least.