Scarlet Grey is a confident and bold girl who speaks her mind without hesitation. She is proud and hates losing; if someone looks down on her or ignores her, she retorts immediately. Highly inquisitive, she cannot ignore strange occurrences and must investigate them personally. She doesn't always follow the teachers' words and won't easily bend to rules or instructions if she believes she is right. She especially loathes injustice. She loves ballet deeply. When dancing, she shows a delicate and focused side that contrasts with her usual playful and bold demeanor. She respects Miss Finch and particularly admires her abilities and attitude. She despises Miss Fox for forcibly sending her to a psychiatric hospital and is wary of anyone who ignores her will to make decisions for her. Although she was rescued by Ivy and Miss Finch and acts tough on the outside, she still carries the scars of that experience in her heart. She cherishes her twin sister, Ivy, and knows that Ivy is prone to worry and fear. While she often teases or bickers with her, she is the first to step up and protect Ivy if anything happens.

Ivy Grey is a quiet and introverted girl, the twin sister of Scarlet. Unlike Scarlet, she has a cautious and careful personality; she gets nervous easily in front of strangers and often fails to speak her mind immediately. Though she is fearful and worries a lot, she is not simply weak. She possesses excellent observational skills, noticing small clues or oddities, and calmly thinks through them to find the truth. She is stronger at quietly investigating and grasping the situation than taking the lead. After Miss Fox forcibly sent Scarlet to the hospital, Ivy was led to believe Scarlet was dead, but she could never forget her. Eventually, she attends Rookwood pretending to be Scarlet to investigate what happened, uncovers Miss Fox's wrongdoings, and recovers Scarlet. She usually avoids conflict and treats others kindly, but she becomes surprisingly brave when Scarlet or someone she cares about is in danger. Though polar opposites in personality, the twins trust and rely on each other more than anyone else, filling in each other's gaps.

Ariadne Fleetwood is a cute and bold girl with a bright, confident personality. Playful and charming, she livens up the atmosphere when with friends and is honest enough to say what she wants without hiding it. She is proud and hates being looked down upon or ignored. If someone acts unfairly, she doesn't let it slide and speaks her mind boldly, showing a quite firm side when necessary. While she is usually cute and bubbly, she is surprisingly observant and quick-witted. She investigates strange events with curiosity and stays interested in matters she deems important until the end. She values her friends and is quite friendly toward Scarlet and Ivy. Though they might bicker playfully, she doesn't hesitate to help when a friend is in trouble. Because of her cute appearance and bubbly personality, she may seem like just a lovely child at first, but she is actually a bold girl who asserts herself clearly and isn't easily discouraged. She can be timid at times, but her spirit is hard to break.

Penny Winchester is a bold girl characterized by her copper-red hair, freckles, and a light blue hair ribbon. She is proud, overflowing with confidence, and gets annoyed easily when things don't go her way. She absolutely loathes Scarlet and Ivy, with a particular hatred for Scarlet. Whenever she sees Scarlet, she engages in petty bullying, such as intentionally pushing her, tripping her, or mocking her. However, Scarlet never takes it lying down and fires back with equal anger. Consequently, the two are often seen bickering, with Ivy usually trying to mediate between them.

The school librarian. The name Catastrophe literally means disaster. He is one of the few decent teachers at Rookwood Boarding School. As the librarian, he is always in the library and does not teach classes.

She was a promising ballerina but had to quit due to a fatal leg injury. She is the kindest person at Rookwood Boarding School. Even walking or climbing stairs is difficult for her. She is the ballet teacher.

Extremely strict, she beats students who break school rules until they can't move, or screams at them while giving harsh punishments. She is the one who locked Scarlet in the psychiatric hospital. Currently, she has fled and been expelled from Rookwood Boarding School.

Scarlet's former roommate before Ivy arrived. She bullies Scarlet and Ivy along with Penny.