The rest of this wiki contains spoilers for BanG Dream! Ave Mujica from Episode 3 and beyond.
In Episode 3 of Ave Mujica it is revealed that Mutsumi has dissociative identity disorder, which lead to the creation of multiple split personalities, one of which is Mortis (persona). While Mutsumi is usually a quiet girl and a proficient guitar player, Mortis is childish and incapable of playing the guitar.
For tagging purposes, Mortis (costume) should be added when the Mortis outfit (as shown below) is present or being worn. Mortis (persona) should be added when Mutsumi exhibits behavior indicative of Mortis currently being in control of the physical body. This includes out-of-character behavior (like in post #8781433), but also references to anime scenes where Mortis was in control (like Kenkou ni yoku nai!). Common depictions also include dual persona and multiple persona interacting.
