Risto is an old demon and a dark magician. By devouring souls he wants to regain the greater power that he once had (which was shrunken down as he got cursed centuries ago). Also he’s certainly a criminal, a mob boss who has many underlings under his command, forging out evil plans in gloomy climes. His huge gut is the result of his greedy nature. He devours not only souls, but also human flesh. He is gross, nasty, smug and most of the time grumpy - and of course - hungry.
Toriki is a human and a magicain (or witch) who is able to directly communicate with souls/ghosts. She uses this ability in her job as an exorcist, though, instead of fighting them and helping humans to get rid of them she cares for the soul’s wellbeing and protection.