Guest's classmate. A top-tier honor student who always gets perfect scores on tests. While he is a serious and diligent worker, he has almost zero social skills, being self-centered and argumentative. He often takes a high-handed attitude toward others and frequently incurs resentment by blurting out whatever is on his mind. He has sharp insight and notices the sisters' worries and personal problems, leading them to solutions each time. He works part-time at REVIVAL to earn money.

The eldest of the five sisters. She calls Futaro "Fu-kun." Among the quintuplets, she is best at math. She has a devilish, teasing personality but also a caring, big-sisterly nature. As the eldest, she is always looking out for her sisters and occasionally offers advice to Futaro, acting like an older sister even though they are the same age. She has a habit of stripping down to just her panties while sleeping. She is a budding actress and was active in small roles while keeping it a secret from her family alongside her studies. (She addresses her sisters without honorifics.)

The second of the five sisters. She calls Futaro "Uesugi" (and occasionally "Fu-kun"). Among the quintuplets, she is best at English. She is a strong-willed, hysterical, and sharp-tongued person, but she is the most delicate of the sisters. She is deeply devoted to them, viewing Futaro as an enemy to protect her sisters and struggling to ensure they all watch the fireworks display together. She has high feminine appeal, enjoying cooking and paying close attention to fashion. (She addresses her sisters without honorifics.)

The third of the five sisters. She calls Futaro "Futaro." Among the quintuplets, she is best at social studies. She has a quiet, cool personality, but she is very honest with people she trusts and has admitted her feelings for Futaro. She is a huge fan of the Sengoku period. (She addresses her sisters without honorifics.)

The fourth of the five sisters. She calls Futaro "Uesugi-san." Among the quintuplets, she is best at Japanese. She is bright, positive, and loves being helpful to others, possessing a good-natured personality that makes it hard for her to refuse requests. She speaks politely to everyone outside her family. She has extremely high athletic ability. (She addresses her sisters without honorifics.)

The fifth of the five sisters. She is serious but clumsy; she has the motivation to study, but it doesn't go well. She is strictly polite to everyone. She is quite a glutton. She is a huge coward and is bad with ghosts and horror movies. (She addresses her sisters without honorifics.)

Futaro's younger sister. A first-year middle school student. In contrast to her brother, she has a bright, honest, and social personality. However, she occasionally makes biting remarks to Futaro. She is in charge of the Uesugi household chores and even handles Futaro's haircuts.

The stepfather of the five sisters. A wealthy man who runs a hospital. A realist who maintains a poker face, but he also has an emotional side that is easily provoked. While he has parental affection, such as worrying about the sisters' poor academic performance, he rarely shows his face at home, citing his busy schedule. He treats his stepdaughters formally, addressing them with "-kun." He is the person who was present at the death of Guest's father and checks in on him whenever possible. He was a senior to Guest's father in high school.

The father of Futaro and Raiha. He works with cameras. He has a rough-and-tumble personality. He is the one who introduced Futaro to the private tutoring job. He was classmates and remains "bad influence" friends with Maruo, the quintuplets' stepfather. A hard worker who has raised two children single-handedly since his wife passed away. He has a wild and casual personality. He was a senior to Guest's father in high school.

The manager of the cake shop "REVIVAL," where Futaro works part-time. He looks intimidating but is friendly and has a gentle personality. He is constantly experimenting to make the shop prosper, and Futaro describes his hungry spirit as something worth emulating. He is friendly toward the Nakano sisters, but he sometimes makes dark jokes, such as saying he will deduct the cost of the sisters' all-you-can-eat sessions from Futaro's salary.