- The characters in the series include but are not limited to:
Akito Himenokōji (姫小路 秋人 Himenokōji Akito)
- Voiced by: Ryōta Ōsaka
He started to take care of his sister when he was young, however when their parents died, the siblings got separated when adopted by two different families. After secretly arranging for years he finally reunited with her and moved out to live by themselves, but only to found that her brother complex since their youth has further developed.
It is later revealed that he is actually adopted by his parents, and has no blood relationship with his sister, but he chooses to keep this as a secret, especially to his sister. He loves to trick Akiko, or watching others trick her. He is quite over-protective to Akiko and makes people around him often comment this as having a serious brother complex, although he and Akiko strongly deny. Since the siblings are now living by themselves, he secretly makes their living by writing romance novels of brother-sister incest under pen name "Kōichirō Shindo", and becoming a famous writer.
He is given the job as student counsel's "vice-assistant of proxy of the secretary", that Arashi happily claims that's a fabricated position for her underground lover; technically, his is the assistant of the secretary, Akiko.
Akiko Himenokōji (姫小路 秋子 Himenokōji Akiko)
- Voiced by: Ibuki Kido
The main heroine and the Saint Liliana High School two year student. Akiko is Akito's sister and has a pretty appearance and polite manners. Akiko was adopted by the Arisugawa family when she and Akito were separated after their parents died. She has very serious brother-complex and is taking very active way to seduce Akito. She is also very proud of her brother complex and makes no effort of concealing it.
She is the secretary of Saint Liliana High School student counsel, but is very reluctant when Akito interacts with other student counsel members.
She doesn't know that Akito has no blood relationship with her and believes that they are twins; according to their deceased mother, Akiko will probably sexually assault her brother once she learn the fact. Even for, now, believing that they are blood related siblings, and respecting her brother's will is the only barrier that stops her from doing so.
She is a fan of Kōichirō Shindo, and is wishing her brother to be as aggressive as the brother in Kōichirō Shindo's novel; however she hasn't notice that Kōichirō Shindo is her brother, and the beginning of Kōichirō Shindo's novel is based on their relationship. It is hinted that Akito actually feels the same way about Akiko but denies it.
Anastasia Nasuhara (那須原 アナスタシア Nasuhara Anasutashia)
- Voiced by: Minori Chihara
Despite being a rival of Akiko, she goes very well with Akito and confessed to him on his first day to school. Although Akito haven't got a chance to answer due to the chaotic situation that the confession brings on afterwards. She demands Akito to call her Anna, but laugh at him for calling a girl as "hole" (which sounds similar to Ana in Japanese) whenever he does so.
Ginbe Haruomi Sawatari (猿渡 銀兵衛 春臣 Sawatari Ginbee Haruomi)
- Voiced by: Asami Shimoda
Akito's childhood friend. Akito constantly tried to get Ginbe to be his friend, but she would keep trying to avoid him. When he visits her house, they finally settle with being best friends. Just as they did, Akito tries to get Ginbe to take a bath with him, not realizing she was actually a girl, due to her acting like a boy and her boyish name.
Despite having a boyish name and talking mood, Ginbei is a girl and secretly has feelings toward Akito. Due to her family issue, she is taught as a boy and cannot work herself, living on only her family's financial support. She later transferred into Saint Liliana High School and became the accountant of the student council. Despite she even dared to move and transfer school to follow Akito, Akito often announces her as his best (normal) f unintentionally, which makes her very disappointed.
She has a habit of mumbling about Akito's unawareness whenever Akito overlooks her feeling.
Arashi Nikaidō (二階堂 嵐 Nikaidō Arashi)
- Voiced by: Eri Kitamura
She has 1st grade on 3rd year and is powerful on leadership, that she can not only handle most student council chores by herself, but also take control of most of the school's activity.
She is bisexual and is very aggressive on lust, that she has over 30 lovers, this also give her a nickname: "predator". However, later in the story she decided to separate peacefully with her lovers and aimed to seduce the entire student council as her harem first.
She is actually the cousin of Akito before his adoption by his parents, she also knows that Akito is adopted by his family, and decided to support him if she can. She knows that Kōichirō Shindo is Akito's pen name.
Arisa Takanomiya (鷹ノ宮 ありさ Takanomiya Arisa)
- Voiced by: Sumire Morohoshi
Daughter of Akito's adopt family and proclaims herself as Akito's fiancee.
Arisa is a genius that despite she is only 12 years old, she is already graduated from a famous university, and her essay are published in multiple famous professional magazines.
Arisa is very proficient at house chores and is a excellent cook. At the end of 4th novel, she moved to live with the student council members as the new manager.
While most student council members refer that the engagement doesn't count due to her age, Akiko still viewed her as a love rival. However, after an argument of who is better for Akito, the two worked together as they noticed that Akito see both of them as his sisters.
She has a habit to go silent and showing emotions on her face, which is so cute that people who see it can't resist accepting her will.
Kaoruko Jinno (神野 薫子 Jinno Kaoruko)
- Voiced by: Megumi Takamoto
Kōichirō Shindo's personal editor. Kaoruko worries that Akito has sister complex and tries to fix the situation. Kaoruko worries about this is because of the way Akito writes about BroXSis doujins. The two main characters in his novel even sharing a similar name to his and Akiko.
She tries at first to seduce Akito at the office when they were alone together, but was mistaken by Akito as Kaoruko imitating a story that he read. Failing at the seduction she goes to the dormitory to find out and confirm her suspicions.
When she meets Akito's sister Kaoruko concludes that it's a Brother with a sister complex and a sister with a brother complex making it even more dangerous.