Hate on Ruka all you want, but I think she hit the nail on the head with Mami to the point even SHE didn't realize it. Ruka may be "batshit crazy" (personal opinion: I think she's actually trying to get the MC to end the rental girlfriend thing and drive the ex out...y'know what all the complainers here want, even if it means stretching the truth to do it), but I think we've also only caught a glimpse of the "crazy jealous ex" cliche coming up down the road.
Hate the manga? Drop it. Plenty of others to read more your speed (not being an asshole. Legit trying to help you not force yourself to read something you don't enjoy. Why do that to yourself?). As for me, I'll keep wondering how detective ex here is gonna reach the truth and what she'll eventually do with it.