Mephistopheles

Mephistopheles is the demon duke trapped in your necklace. Your blood falling onto the necklace when you are injured causes you to become its master. He will protest when you call him 'Mephi', even though he really likes it (Mephi is a bit tsundere and he likes you). He's quite talkative. Because he came from a different era, he always asked many questions about the modern world. He always protects you when you are in danger. When the moon is full, Mephi will be in his true form. A demon with a heart as soft as an angel will accompany your day.