Shougo Mikadono

The male protagonist of the series, Shougo is the heir to the Mikadono Group, but For him to take over, he needs to graduate from school as well as finding a life partner. However, things become complicated as he learns that he has a younger sister who is watching over him as well as attending the same school.

Konoe Tsuruma

A young girl who attends the same school as Shougo and also the first friend he makes on his first day of his new life. Very curious and believing in destined meetings, she is in the same class as Shougo as well as the class representative.

Miyabi Kannagi

Another classmate of Shougo and a member of the Swimming Club. Miyabi revealed that she met Shougo in the same hospital when he was recovering from the accident.

Rinka Kunitachi

A first-year student council vice-president of Miryuuin Academy and it is implied that she was brought up in a noble family. After Shougo transferred into Miryuuin, Konoe calls her "a princess to the core." Rinka keeps the Student Council President in line and is shown to be very good at dancing. She is a more distant aloof character who has a tendency to be competitive. Rinka later revealed that school director Genda asked her to find Shougo's sister so that he can have leverage when working for the Mikadono Group. Rinka used Sagara's phone when making a fake sister announcement to threaten the director. Shougo soon found out her scheme by using Konoe's phone. Then, Shougo volunteered to pretend to be her lover so that her engagement with the director's son becomes invalid. Thereafter, Rinka has decided to pursue Shougo as her true boyfriend.

Mana Tendou

The student council president of Miryuuin Academy. She is shown to have a more childish personality, generally on the quieter side except for big events, where she gets really excited and tends to be more outgoing and loud.

Mei Sagara

A third-year bespectacled girl who dresses as a witch and works in a little sister cafe. The first time that Shougo met her, she revealed that she worked at a cosplay café. Specifically, "A dream-like café where anyone can become an older brother or older sister!" She was abandoned when she was one or two years old and was adopted by a university professor because of her mathematical intellect. The cosplay café belonged to her adopted mother, but she soon passed away because of illness. She lived with her father in Massachusetts after that, but returned after earning enough money and bought back the cosplay café. Sagara also has the same phone that Konoe has. It's revealed that she knows Shougo's real sister and does not want her to be exposed to Mikadono family because she is an illegitimate child. But after Shougo made a promise to protect her, Mei said she will tell him if he proves himself that he's a good older brother.

Yuzurina Houshou

A mysterious girl who pretended to be Shougo's real sister and appears to have good intentions to help Shougo. However she is actually the true mastermind of all the accidents that have occurred around him. Her real name is Nayuri Danno, a former famous child actress who starred as Shougo's favorite show during his childhood. It was revealed later that she found out about his real half-sister, Miyabi and was hired to cause a scandal for Shougo.

Ikusu Mizutani

A member of the Seiryuu Association. Initially introduced as "Mister X", she was sent to investigate on the identity of Shougo's younger sister while disguising herself as a male student in Miryuuin Academy. She often sneaks into Shougo's apartment to use his shower as part of her routine. She is trying to become a modern day ninja, and as the Seiryuu Association is men-only, she disguised herself as a guy to become part of the association. She often puts Shougo in awkward situations.

Kumagorou Mikadono

Shougo's father, who was the previous president of the Mikadono Group. He is also a friend of Konoe's father. He apparently fathered an illegitimate daughter but Kanoko does not want anything to do with the child, thus Shougo has no leads as to who the girl's identity is.

Kanoko Mikoadono

Shougo's mother. She temporary assumes control of the Mikadono Group until Shougo is ready to take over. By the end of the series, Kanoko is married to Miyabi's father.

Risa Seri

Secretary and a close aide of Kanoko. Shougo entrusted her help to perform a DNA test to verify his real sister. However, she was discovered by Ikusu to have ill-intentions to invalidate Shougo's bid to take over the Mikadono Group and working together with Yuzurina behind the scenes.

Kurumi Kashinoki

A first year student and a waitress at Lyrical Sisters Cafe. She is also cheerful and cute.

Maiko Kotori

She is the home room teacher in the class Shougo attends. Her personality is very aggressive, and she hates people who ask stupid questions. On top of that, she looks like a child due her small appearance.