Dr. M. Butler (died 2020) was the warden and chief psychiatrist of Arkham Asylum.
In 2020, after Mouse was arrested by the Crows and sent to Arkham again, Butler locked him into isolation under the asylum's underground. That night, Alice broke inside his office and threatened to kill him to avenge her brother-figure. However, she eventually spared him and instead knocked him out to steal the key to access Mouse's cell as she had promised her sister.
Over the following weeks Butler repeatedly tortured Alice via electroshock. Later, while having a session with some patients, after Mouse spoke about his past he encouraged Alice to do the same but is interrupted by Tommy Elliot, who started to speak obsessively about Bruce Wayne. Butler tried to calm him and have him change topic, to which Elliot replied by insulting the beliefs of every other mental patient until he was dragged away.
The following day, while Butler was preparing to have an electroshock therapy session with Mouse, while he was distracted, the latter knocked out the guard and stabbed Butler in the chest, before he could yell for help, killed him and took his place.
For days, with Mouse using a removable mask, residents and employees of Arkham thought Butler was still alive. Elliot thought that Butler was working with Alice when Mouse was with her when she approached him, until Mouse removed his mask. Nurses, like Janet, talked with Mouse not knowing the difference.
Ultimately, however, the guise was destroyed when Alice unmasked Mouse and threw it into the flames during a fire in Arkham.