Yukiko-sama appears at the end of episode 3, where she and her companion plots about Sakura and her friends mahou group.
Yukiko is a kuudere, and is heavily depicted as evil by her colour scheme, eyepatch and attitude.
It is also heavily implied here that Yukiko is going to be involved in a relationship of some sort with Ruko.
She has a small bunny-like companion.
She is voiced by LPSkittycat100/LilacTheBatt.