Machikado Mazoku

Yoshida Yuko was living a perfectly normal (if poverty-stricken) life, but at the age of fifteen, she suddenly sprouts demon horns and a tail! Her mother explains that she's actually a demon named Shadow Mistress Yuko, once sealed away and cursed so that her family would remain poor for generations. Now Yuko can break that very curse if she defeats Chiyoda Momo, a magical girl who goes to her school…but Momo is stronger than she is, knows how to manage magical powers, and is so freaking nice that she's actually helping Yuko out of scrapes. Yuko's too weak and uncoordinated to straight-up kill Momo, even if she wanted to, but she can't just be buddies with this generous magical girl, either–she has to break the curse on her family somehow. What's a demon girl to do?

(Source: Seven Seas Entertainment)