she was the black cat

she was the new black cat yes

She was the girl that switched Mitsuya(the black cat) because she was murdered. Well, basically she murdered her so that the manager will take her instead.

In an earlier episode, the new black cat who replaced the mystery kiss member who died said that she found out she was only 1 spot away from being in the group.

Because of this, and the fact we now know she killed the other mystery kiss member, it's clear she did this knowing if she killed another member of the group, she would get in.