In episode 3 of Ave Mujica, Mortis is introduced as an alternate personality of Mutsumi, who is revealed to have Disassociative Identity Disorder, as Mortis takes control of Mutsumi's body while Mutsumi is, in her words, "asleep". Compared to Mutsumi, Mortis is characterized as more extroverted and childish, and is also prone to more outward and often theatrical expressions of her emotions, as well as copying other's behaviors.
