A serialized manga proves popular enough to continue and introduces a previously unseen character.
Whatever will they think of next?
Maybe they'll think of an unseen character whose "sympathy factor" won't rely on a backstory that doesn't really work (for me) if it was never mentioned. She could have had other reasons for being antagonistic towards Onee-san that didn't revolve around her supposedly being with friends with Mai-chan.
Or maybe they'll think of a new character who doesn't follow the predictable path of:
1. Being a brat.
2. Reveal the reason for being a brat.
3. Get along with everyone by the end.
And maybe even double-down on it and have her be a part of a love triangle between Mai-chan and Onee-san (although I didn't interpret it this way, but let's see where volume 3 takes us).