Haru and Carol--that's all Ichinojou needed. Mareena is a HARDCORE example of excess. Haru and Carol are both lovable in their own ways, but Mareena is exactly the opposite: her most loathsome traits (the first two don't even have any negative traits worth speaking of, BTW) are almost constantly on full display, and she has few to no redeeming traits to speak of--even when she takes off that godforsaken mask.
It's like she's solely there to create difficulty and stress for the MC.