Ruby is a character who first appeared in the episode Rotkäppchen. She has lost her Necromancy powers that she uses to slay the Why-wolves, and wants to retrieve them by her grandmother. But instead Marceline helps her out.
She bears much resemblance to Little Red Riding Hood (Rotkäppchen in the Grimm Brothers, which also explains the title). She has brown hair and black dots as eyes. Although not seen she said she wears a black school uniform underneath her coat.
Ruby is a Necromancer which allowes her to preform dark magic. She is able to create weapons with the red essence that is floating around her. She mostly uses, dagger-like weapons and spike knuckles to fight. But she is also seen creating a scythe and preform slashes releasing red sharp rose petals, but this does take a lot of her energy.