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 plainest appearance and personality. She uses the word repeatedly and sometimes appears even offended if someone else tries to argue that she isn't plain. 

Here are some voice examples: https://youtu.be/ml6YEge1OhQ