Little Demon is the personification of Soul's insanity brought on by the Black Blood he was infected with.
Maka during a soul resonance with Little Demon.
In the series it appears in Soul's mind wearing a black suit and tempting Soul with significantly increased power in exchange for his sanity. Soul usually refuses this offer unless he or his partner's life is on the line. When accepted, Maka's appearance becomes insane during a soul resonance and she fights with little regard for own safety.