Mitty is an amorphous creature that lives with Nanachi on the 4th Layer of the Abyss.
Mitty used be a regular human girl in the past, and her current appearance is the result of the ascension from the 6th layer forced on her by Bondrewd. Her condition caused her soul to be imprisoned within the amorphous carcass which was previously her body, though she's unable to communicate in any significant way. In her current form, she is also "immortal," in that stab wounds or other similar injuries cannot affect her whatsoever as, while she will feel pain, she will soon regrow.
At the end of volume 3 (and the end of season 1 in the anime), in order to avoid leaving Mitty all alone, Nanachi ultimately decides to have Regu shoot and kill her with his Incinerator—which "erases" and rewrites rules of the Abyss—so she could finally be at peace.
