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.
A flashback reveals that he was born female, but was unable to reconcile his feelings of discomfort towards his gender. As such, he 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 towards others. Mutsuki uses the personal pronoun ore (俺), usually reserved for male characters that are tough or aggressive which is fitting for Mutsuki to use as he is a transgender male.