One year ago, Yeorum Han's older sister Gyeowul was found dead in a strange costume, having apparently jumped from the top of the school. Yeorum, now a 12th grader just like her sister was, is understandably having a difficult time. On her first day, she falls asleep in class, and the teachers decide to just leave her alone. She wakes up at 3 AM in an empty school, having missed every bell.
Then she stumbles across her (male) biology teacher dressed like a Magical Girl, fighting giant floating goldfish.
Yeorum is forced to become a magical girl like her sister, fighting dangerous monsters which appear to be caused by her own subconscious. But between an insane biology teacher, a perverted fellow magical girl, and a strange woman who appears to be her dead sister Gyeowul, the monsters are the least of her problem.
- Abusive Parents: Eunmi is constantly belittling her daughter, even going as far as refusing to call Aran by her first name (She calls her "F Student" instead) She gets better once her future self confronted her, however
- Sohui's parents. While it may come off as Innocently Insensitive on their part, they managed to hard wire in her brain that the only way to make them happy was by studying hard (keep in mind that she's a Kindergardener forced to take High School level Algebra.) They even locked her in a closet just so she could do her work. it eventually gotten to the point where when Future!Sohui met her Past!Parents, she tried to kill them by burying them in schoolwork, and had it not been for Aran, she probably would've succeeded.
- All-Loving Hero: Gyeowul loved her sister dearly, despite Yeorum never being anything but trouble for her. It's even implied that Gyeowul knew Yeorum was faking being an Ill Girl for the attention, but she loved her anyway.
- Always Identical Twins: Sort of. Yeorum and Gyeowul are not twins, but they look nearly identical despite being a year apart. The "Gyeowul" running around is actually an older version of Yeorum, and not even Yeorum notices.
- Bad Future: The future the older Yeorum and the rest of her group are from turns out to be this.
- Blind Without 'Em: Namgung's vision is shown to be more or less a blur when he's not wearing his glasses.
- Chekhov's Skill: Yeorum's method of transformation involves stabbing herself with the magical hair-pin. During the final battle against Gyeowl's spirit (Who was wearing the pin as a giant sword on her arm), she stabs Yeorum.
- Covert Pervert: The reason Namgung still goes around dressed like a magical girl despite having two actual magical girls around? Because Aran insists on it.
- Education Mama: Aran's mother pushes her hard to succeed academically, even making her study algebra when she was still a small child.
- I Hate Past Me: The Yeorum from the future is well aware that pretty much everything bad that ever happened to Gyeowul, and most of the bad stuff happening now, is Yeorum's fault. She finds her disgusting.
- A milder version with Aran. The Aran from the future arranges a meeting between Sohui and Present!Aran so Present!Aran doesn't walk the same path that she did, catering to her mother's strict standards and sacrificing happiness in the process.
- Ill Girl: Yeorum is a psychological version during the current story, and in flashbacks she was doted on by her parents for physical problems. She was faking at least some of it for the attention.
- Les Yay: Played for laughs. In chapter 37 Yeorum's apology to Aran is a grand, sweeping gesture complete with balloons and sparklers in the shape of a big heart. Aran asks if she's proposing to her twice.
- My Greatest Failure: Namgung blames himself for Gyeowul's death, and wishes more than anything that he can fix it—but the wish that the pin grants can't be used to revive the dead. He can, however, use it to turn back time, but he can't turn it back to before the moment where she died. He's still tried a dozen times.
- Never the Selves Shall Meet: Different copies of the same object cannot survive in each other's presence for long. Inevitably, the "younger" one, from the current timeline, will survive while the older one is destroyed. This applies to people as well, with the added caveat that the older copy can survive as long as the younger one doesn't realize they're the same person.
- Running Gag: Yeorum getting an eyeful of manparts due to a wardrobe malfunction, nude transformation sequence, or what have you.
- Theme Naming: "Gyeowul" means winter and "Yeorum" means summer, so it's often assumed they were born in winter and summer respectively. The truth is that they were both born in December, albeit a year apart. Yeorum was just named to fit Gyeowul's theme.
- Transformation Sequence: Usually naked, always lampshaded. This includes the boys, with the author noting that the lower half is obscured because he doesn't want to draw it. It's not obscured for Yeorum.
- Transformation Trinket: The pin, a magical artifact that can transform anyone into a magical girl (including boys, though their outfits are suitably masculine). The cubics, little pieces of the pin that broke off, can be used for the same purpose, though they're less powerful.