The Ultimate Cosplayer. Tsumugi is an honors-type student who speaks in a docile and gentle way, but she becomes very passionate when she talks about her favorite series or cosplays, having a habit of slipping infamous quotes from various sources into the conversation. Being a self-proclaimed introvert, she also tends to ignore people while she's in her thoughts, no matter how many times they try talking to her, and appears rather distant at times. While Tsumugi is the Ultimate Cosplayer, she only enjoys making costumes, not wearing them, as she is not fond of people looking at her. However, she hates using cosplayers who care more about standing out than the love of the characters, so she wears her costumes as well but with love. She deeply loves cosplaying as she states that cosplaying is an expression of love. She has a deep respect for fictional characters and gets very offended if they are disrespected. Tsumugi is somewhat timid, and she is usually the first person who screams whenever something scary happens.

After being revealed as the mastermind of the Killing School Semester in Chapter 6, she reveals herself to not be that much different from Junko Enoshima herself, except not possessing any special analytical prowess and superior charisma that Junko had. She is manipulative and cruel.