Machikado Mazoku

Yuuko Yoshida wakes one morning with horns, a tail, and some deeply inconvenient family history. Her mother reveals that their impoverished household descends from the Dark Clan, cursed long ago by the rival Clan of Light. To break the curse, Yuuko is told to defeat a magical girl and offer her blood to their ancestor. The obvious target is her classmate Momo Chiyoda, except Momo is vastly stronger, oddly helpful, and far too kind to behave like a proper enemy. Adopting the grand title Shadow Mistress Yuuko, or Shamiko, Yuuko begins a campaign against Momo that produces more training sessions, borrowed favors, and accidental friendship than evil triumphs.