He is a Rank 3 Ghoul Investigator, an artificial one-eyed ghoul, and a member of CCG’s Quinx Squad.
Tooru is kind with a gentle heart and seems to be shy with little self-confidence. Having a constant case of Anemia, ironically, Mutsuki feels repulsive when he comes into contact with blood. Because of this anxiety, he faces discouragement as a ghoul investigator, to the point where he feels as though he isn’t even needed for the job. He expresses frustration with his own weakness, feeling anger at his lack of ability compared to his comrades. When not working, he is shown to enjoy reading books and is the one often left to relate information to Haise Sasaki. As such, he is shown to be a reliable and hard-working individual, following orders in contrast to the more troublesome or lazy members of his team. Though lacking in confidence, he shows a strong desire to help others and does not hesitate to place himself at risk to protect the innocent.
He shows great respect to Sasaki and Dr. Shiba, as well as other Investigators even when treated poorly by them. He is also a confidant to his superior, as he always reassures him when he is feeling down, thus showing a great fondness and admiration for Sasaki.
A flashback reveals him to have been born female, but unable to reconcile his feelings of discomfort toward his birth sex. As such, he has began living as a man and uses highly-masculine personal pronouns, even when such informal language is not socially appropriate. It contrasts strongly with his gentle personality, and his consistent respect toward others.
Near the end of the Auction House Raid he was finally able to activate his kagune.
He later becomes a Rank 1 investigator.
It’s later revealed that his father sexually abused him, which eventually lead to his having a mental breakdown and brutally murdering his family. After this incident, however, he repressed this deranged personality of his, which has only occasionally resurfaced, primarily in cases, where his life was in danger such as the auction house or when he was held captive and tortured by the ghoul Torso.