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. She tends to call herself and everything that she has done "plain," as she believes that among all of the students, she is the one who has the plainist appearance and personality. She uses the word repeatedly and sometimes appears even offended if someone else tries to argue that she isn't plain. She understands that some people might not recognize her as the Ultimate Cosplayer since her plain appearance conceals her talent.

Despite her plain and docile attitude, certain hints suggest that she may actually be quite perverted, as she is a bit stereotypical fan of anime and manga. After the time limit motive was made apparent in Chapter 1, she was saddened by not being able to hide her "special art commissions" and "that pillowcase" she bought before she dies. She also at one point, in an optional dialogue conversation, wonders if she could wipe her entire hard drive at home from within the Ultimate Academy, worrying about the pictures that were saved on it, perhaps hinting at an interest in pornography, hentai, or something similar to them.