Correct me if I'm wrong, but I THINK this is Marisa's Witch.
DESDEMONA The Stardust Witch. Her nature is power. She's so boringly mediocre to the point that her existence gets called trash and junk. However, she won’t admit that, she just keeps working hard to improve herself, in order to become a special existence someday. If you show her a flashy attack, even if it's just pretend, she will realize her powerlessness all on her own and lose the will to fight.