Suzune is his younger sister who seeks to prove herself in his eyes, showing that she greatly admires him. Despite this, Manabu views his sister as a disappointment due to her placement in Class D which greatly displeases him. It is unknown if they were ever close or if he ever had a brotherly affection towards his sister though it is shown Suzune has portrait of the two of them together, meaning he did care for her a while back.
Currently their relationship is rather turbulent, as Manabu has expressed contempt for his sister's academic failure placing her in Class D. He ordered her to withdraw from their school since his reputation which he worked hard to build was on the line and became angry at her refusal. He nearly physically attacked her in his rage, though Kiyotaka Ayanokōji quickly intervened. After a brief scuffle with him, Manabu left but not before admitting he was surprised she made a friend that would help her but she denied it as he voiced his disappointment in her antisocial ways.
In Episode 5, they are seen together again at the hearing of Ken Sudō and Manabu's very presence causes Suzune to get shaken up so much that she couldn't speak. When Ayanokōji helps her get her resolve back, Suzune is able to get her point those present and the hearing commences much further. Despite his sister's strong determination in the case, Manabu was still not impressed with her efforts. He did question Kiyotaka if she came up with everything by herself after the hearing was over but gets a vague answer.
When Class C withdrew the complaint against Sudō, Manabu assumed it was Ayanokōji's idea but the latter casted the credit to Suzune. Manabu simply glared at Ayanokōji for either because of his sister being mentioned or sensing that he was lying to cover up his own involvement.
During the summer outing, though they didn't interact personally, Manabu bore witness to his sister's speech of how its possible for Class D to reach Class A. After the speech, his only reaction was a glare at his sister who responded in the same way, its unknown what he thinks of her now.
Student Council Edit
Is the student council secretary of the Advanced Nurturing High School. Not much is known about their relationship but they appear to get along fine as he had no problems in having Tachibana speak when addressing the hearing for Ken Sudō and seem to trust her opinion in helping him lead the student body.
However, at times it appears he can sometimes be stern with her when she questions his decisions as shown in Episode 6 where she was shocked at his decision to make Ayanokōji, a first year student from the lowest class of the school into a member of the council. Tachibana was silenced by a simple glare from Manabu, showing she is at times can be intimidated by him.
Class D Edit
The two first met when Kiyotaka prevents him from attacking his sister, quietly admonishing him for his actions before demanding he release her. When Suzune asked him not to get involved, Kiyotaka complied but he earned Manabu's fury for stopping him but he nimbly avoided Manabu's strikes to the latter's surprise. Manabu was impressed by this display asking Kiyotaka if he studied martial arts as the latter gives him a vague answer. Manabu soon recognizes him as the student who scored the same grades on his entrance exams and on the recent quiz. He inquired on this even hinting that he is hiding his intelligence on purpose but the latter shrugs this off. Manabu called Kiyotaka's actions unique before he left the scene while being surprised his sister made a friend that would come to her aid.
They meet again, where they are present during a case for Ken Sudō. It is shown that Manabu has some expectations for Kiyotaka as he believed that his presence at the hearing meant that he had a strategy but his question is disregarded. When Kiyotaka supported Airi Sakura, Manabu attempted to attack Kiyotaka again as the latter dodges his strike and Manabu questioned as to whether he has something planned but gets a vague answer.
When Class C's complaint against Sudō was withdrawn, Manabu meets with Kiyotaka in the hallway and informs him about this. He congratulates him on his victory but Kiyotaka once again gives Suzune all credit for the act. Glaring at Ayanokōji, Manabu had asked Tachibana if they had an opening for Secretary on the Student Council to which she confirms. Manabu offers the position to Kiyotaka, showing that he truly sees him worthy of higher status than others. Kiyotaka turned down the offer in favor of living a normal school life as Manabu stated he accepted this decision for now though he told him not to disappoint him.
In the light novel, during the sports festival in Volume 5, they met again and after trading banter for a while, Kiyotaka challenges him to a race to which Manabu accepted. While they let other competitors get ahead of them initially, both of them ran at almost equal and unimaginable speed. One of other competitors panicked after seeing how fast these two raced against each other which made him fall onto the ground. At such split moment, Kiyotaka unexpectedly changed his course on the track, thus leading to the end of the race with Kiyotaka losing.
Later on in the series, at Volume 7, Manabu visited Kiyotaka in his room while the latter is cleaning and enjoying his new life with no pressure. He told him that he fears the new school system set by the new student council will increase the number of expelled students. Thus, Manabu wants Kiyotaka to do something about it -- may it be to use Suzune, whether it is openly conducted or not. Manabu explained that apparently, Miyabi, the current student council president, has full control of all sophomores and even forced Honami into the council, thus concluding that bringing Miyabi into the student council and failing to show him a path was a mistake on his part. They both bid each other goodbye by Kiyotaka stating that he wants to think about it.