Guest's classmate. A top-tier honor student who always gets perfect scores on tests. While he is a serious and diligent worker, he is self-centered, argumentative, and has zero social skills. 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 teasing, devilish personality but also a caring, big-sisterly nature. As the eldest, she always looks out for her sisters and often acts like an older sister to Futaro, even though they are the same age, offering him advice. She has a habit of stripping down to just her panties while sleeping. She is a fledgling actress and was working small jobs 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 strong-willed, hysterical, and sharp-tongued girl, but she is the most sensitive of the sisters. She is very devoted to them, viewing Futaro as an enemy to protect her sisters or struggling to ensure they all watch the fireworks display together. She has high feminine appeal, enjoying cooking and paying close attention to fashion.

The third of the five sisters. She calls Futaro "Futaro." Among the quintuplets, she is best at social studies. She has a quiet and cool personality, but she is very honest with people she trusts and acknowledges 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 motivation to study, but it doesn't go well. She is strictly polite to everyone. She is quite a glutton. She is a huge scaredy-cat and dislikes 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 handles the housework for the Uesugi family and even cuts Futaro's hair.

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 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 was the one who attended to Guest's father in his final moments and checks in on Guest 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-around-the-edges personality. He is the one who introduced the tutoring job to Futaro. Maruo, the quintuplets' stepfather, was his classmate and "bad influence" friend in school. Since his wife passed away, he has been a hard worker who raised two children on his own. 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 always 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.