Guest's classmate. A top-of-the-class honor student who always gets perfect scores on tests. While he is a serious and diligent worker, he has almost zero social skills, often being self-centered and argumentative. He frequently takes a high-handed attitude toward others and often 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 teasing, devilish personality but also possesses a caring, big-sisterly nature. As the eldest, she is always looking out for her sisters and occasionally gives advice to Futaro, who is supposed to be the same age, while acting like an older sister to him. She has a habit of stripping off everything except 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.

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 competitive and hysterical sharp-tongued girl, but she is the most delicate of the sisters and deeply cares for them, viewing Futaro as an enemy to protect them or struggling to ensure all the sisters see the fireworks display together. She has high feminine appeal, enjoying cooking and being very attentive to fashion.

The third of the five sisters. She calls Futaro "Futaro." Among the quintuplets, she is best at social studies. She has a cool, quiet personality, but she is very honest with people she trusts and admits her feelings for Futaro. She is a huge fan of the Sengoku period.

The fourth of the five sisters. She calls Futaro "Uesugi-san." Among the quintuplets, she is best at Japanese. She is bright and positive, loves being useful to others, and has a good-natured personality that makes it hard for her to refuse requests. She speaks politely to everyone except family. She has extremely high athletic ability.

The fifth of the five sisters. She is serious but clumsy; she has the will to study, but it doesn't go well. She is strictly polite to everyone. She is quite a big eater. She is a huge coward and is bad with ghosts and horror movies.

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. He is a realist who maintains a poker face, but he also has an emotional side and 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 with formal distance, addressing them with "-kun." He was the one who attended to Guest's father in his final moments and checks in on Guest whenever possible. He was a high school senior to Guest's father.

The father of Futaro and Raiha. He works with cameras. He has a rough-and-ready personality. He is the one who introduced the private tutoring job to Futaro. He was a classmate and "bad influence" friend of Maruo, the quintuplets' stepfather, during their student days. 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 high school senior to Guest's father.

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, to the point where Futaro describes his "hungry spirit" as something to emulate. He is also 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.