Minor characters
Yuiko Hawatari (羽渡唯子, Hawatari Yuiko)
Voiced by: Kana Ueda (Japanese); Shannon Settlemyre (English)
Yuiko is Ritsuka's classmate. Despite her ditzy, bubbly exterior, she is sharp and goodhearted. Ritsuka at first found her annoying. On Ritsuka's first day in school, she immediately is overcome with awe and a fascination for him, and has feelings for him. At the prodding of her mean-spirited classmates, she befriends him, although he is very reluctant to accept her because he thinks she is weird and unintelligent. As Ritsuka overcomes his initial impressions of her, they become good friends.
Yuiko and Ritsuka's friendship greatly irritates their classmate, Yayoi. He had been recently rejected by Yuiko, although rather sorrowfully, claiming she will only date boys taller than her. Much to Yuiko's dismay, Ritsuka insists their friendship is platonic and it, indeed, remains that way. However, in two separate confessions to Hitomi Shinonome, Yuiko professes her love and devotion to Ritsuka. Ritsuka seems to trust Yuiko, despite his own issues with that kind of bond.
As the series progresses, a rather unusual, yet harmonious, friendship is struck between Yuiko and Soubi. Although it seems their personalities would clash, they have a unique understanding of each other.
Yuiko is characterized by her large breasts and considerable height. At 168 cm tall, Yuiko is much taller than Ritsuka and her other classmates and is often mistaken for being older than she actually is.
Kio Kaidou (海堂貴緒, Kaidō Kio)
Voiced by: Ken Takeuchi (Japanese); Jack W. Gibson (English)
He is a good friend and current classmate of Soubi's. They are both art students. They study the Japanese art of flowers. He describes himself as Soubi's only friend. Most of his early dialogue is filled with flirtations and passes at Soubi, despite being gently ignored or indirectly rejected each time.
Despite his early presence as a character who exists solely to create a lighter atmosphere among a very dark story, he gradually gains depth as the story progresses. He becomes a loud voice that champions pacifism and self-worth. This contrasts strongly against Soubi's submissive nature that willingly accepts violent treatment and strongly desires to be completely controlled, albeit from only very certain people. More significantly, Kio's views are shared by Ritsuka. Together, they provide a very black-and-white atmosphere that allows only one line of thinking.
It was recently revealed that Soubi only became interested in Kio because of a tattoo on his back, and a vow that he took when he got it.
He has a daughter that looks to be around ten years old; she notably has no cat ears. A twin sister also appeared but he had no knowledge of her.
Hitomi Shinonome (東雲瞳, Shinonome Hitomi)
Voiced by: Mamiko Noto (Japanese); Jaymie Krasinski (English)
She is Ritsuka, Yuiko and Yayoi's teacher. She is very naive and innocent, and painfully aware of these characteristics. At twenty-three years old, she is teased and ridiculed for retaining her ears (and therefore her virginity) to such an age. She is visibly embarrassed, but as the story progresses, she gains confidence and pride over making her own choices.
Hitomi is greatly troubled by Ritsuka's mother and the injuries he sustains from her, and greatly desires to involve herself in the situation to make a change, despite her fears and her colleagues' dissent.
Her very caring and naive nature clashes spectacularly when confronted with Soubi. He brusquely rejects her and says he doesn't care for older women, although she has never made a romantic move towards him and is no more than three years older than he is. Despite his cold treatment, Hitomi seems to have had some feelings for Soubi. His brash actions are mysterious—the only discernible reason being a general hatred towards teachers as a result of his experiences with Ritsu Minami.
Although he has rejected her, it can be noted that Soubi enjoys tormenting her to some degree.
This kind of behavior towards Hitomi may have stemmed from jealousy. Soubi refers to Hitomi as 'childish', despite her being three years older than he is. Yet he still respectfully calls her "Shinonome-sensei." The only way he degrades her is by her sexuality, possibly because Hitomi has remained a virgin while he lost his ‘ears’ at a much earlier age. In comparison with his extreme dislike of his own teacher figure, Ritsu, Soubi still manages to treat Hitomi as a subordinate, constantly belittling her.
Misaki Aoyagi (青柳美咲, Aoyagi Misaki)
Voiced by: Wakana Yamazaki (Japanese); Ashley Thrill (English)
She is the mother of Seimei and Ritsuka. She has an important influence over her sons. Being emotionally and mentally weak, Misaki has very little grip on reality, and often hallucinates, particularly after certain events in volume 5. She is abusive and violent and blames Ritsuka's personality change for the reason Seimei "left", demanding that she wants her former Ritsuka back when she beats him. To many, she is seen as a monster, but in volume 5, she has inner dialogues of her own, allowing the reader to witness a more human, fragile side to her behavior which she cannot control.
Yayoi Shioiri (塩入弥生, Shioiri Yayoi)
Voiced by: Jun Fukuyama (Japanese); Michael Yeager (English)
Yayoi is another classmate of Ritsuka's. He is shown as being socially awkward and with low self-esteem, and feels great jealousy towards Ritsuka. Yayoi admits that he loves Yuiko, but when he confessed to her, he was rejected for being too short. Yuiko claims that she only dates boys taller than her, but the truthfulness of that statement is called into question when it is painfully obvious she loves Ritsuka, who is barely taller than Yayoi.
Despite his jealousy of Ritsuka's close friendship with Yuiko, he is encouraged by Ritsuka's adamant claims that their relationship is purely platonic. He makes a firm statement he will not give up on Yuiko, and spends more time with her than ever.
It is notable that in the manga, Yayoi has a rather small role. He is absent for large parts of the story, and has only a small amount of dialogue. In the anime, they utilized his character much more to enhance the would-be Yayoi-Yuiko-Ritsuka love triangle, and to inject comic relief into a dark story.
Nagisa Sagan (目渚, Sagan Nagisa)
Voiced by: Sanae Kobayashi (Japanese); Ashley Thrill (English)
She is an easily angered woman who conducts experiments and creates humans who cannot feel physical pain. Although Nagisa is very young in appearance, it is later revealed that Nagisa is over 30 years old. Her background, and affiliation are unknown. She had a sister, but what has become of her has not yet been confirmed. She created Youji after the image of her younger sister, perhaps in remembrance or to ease the pain in her heart, but exact reasons are still unknown. Her over-the-top behavior and strange lifestyle leaves her alone and alienated. She befriends Nana, and shamefully confesses that she is her only friend.
Her relationship with Ritsu is an intriguing one. She puts up a front of hating him deeply, and wishing for nothing more than to see him fail. However, there are small moments that reveal possibly more intimate feelings, as she blushes deeply whenever he is near her or touches her, though she steadfastly denies caring for him. Starting in volume 3 and finishing in volume 4, she makes a bet with Ritsu that her older Zero pair, Yamato and Kouya, can beat Ritsuka's Soubi. Ultimately, Yamato and Kouya are defeated, and Nagisa's sibling rivalry-like attitude towards Ritsu continues. Also when Seimei gouges out Ritsu's eyes she goes crazy, showing more than she probably intended of her true feelings for him, but still denies it when her Zeros make fun of her over it.
Natsuo Sagan (目奈津生, Sagan Natsuo)
Voiced by: Mitsuki Saiga (Japanese); Ashley Crist (English)
He is the fighter of the male Zero pair. He is one of Nagisa's experiments, and presumably the second generation of Zeros. He first makes an appearance in volume 2, when he and Youji corner Soubi when he's alone. When Soubi wins the battle, Natsuo tells him that he and Youji have no place to return to if they lose a battle. Soubi offers to let them live at his apartment, and Natsuo warily accepts. Natsuo seems to care for Soubi and Ritsuka, and in turn, does nothing to harm them.
In one of the Zero-sum chapters, they are told to defeat Seimei. But because of the other's unusual power with words, the Zero team was rendered practically useless. In the past Natsuo pulled his own eye out but he felt nothing, because the Zero pair feels no pain.
Youji Sagan (目瑤二, Sagan Yōuji)
Voiced by: Hiroyuki Yoshino (Japanese); Shannon Settlemyre (English)
He is the sacrifice of the male Zero pair. He is the more cruel and brutal of the two; however, they are both rather blunt and cold towards others. Like Natsuo, Youji seems to care for Soubi and Ritsuka, and in turn, does nothing to harm them.
Nagisa is unashamedly biased towards Youji, often ignoring Natsuo. According to Youji, this is because he bears a significant resemblance to Nagisa's younger sister.
In one of the Zero-sum chapters, they are told to defeat Seimei. But because of the other's unusual power with words, the Zero team was rendered practically useless.
Kouya Sakagami (坂上江夜, Sakagami Kōya)
Voiced by: Rie Kugimiya (Japanese); Renale Hailey (English)
She is the fighter of the female Zero pair. Until they met, both pairs believed they were the only Zero pair to exist. Both teams are experiments created by Nagisa at different times, and the female pair is presumably the first generation. Despite their unusual background, both Kouya and Yamato attend high school, although different ones.
In an interesting demonstration of the series' fictional world's rules and expectations of society, Kouya wears fake cat ears despite not being a virgin. Yamato often teases her about it, particularly because Yamato doesn't try to hide her status. They lost their "ears" to each other. Shortly, after they first met, Kouya slit her wrists just to "know how it would feel", since Zeroes can't feel pain.
Yamato Nakano (中野倭, Nakano Yamato)
Voiced by: Yumi Kakazu (Japanese); Li Clapp (English)
She is the sacrifice of the female Zero pair. In contrast to Kouya's solemness, Yamato has an outgoing, vivacious personality. Their relationship is the focus of a mini-arc in volume 4.
The relationship Yamato has with Kouya is a passionate and dedicated one that becomes unexpectedly stressful for Yamato in volume 4. Her marking, which she interprets as physical proof of her relationship with Kouya, slowly fades without warning. In a blatant reference to the homosexual nature of their relationship, both girls' markings—a 0 with a line through the middle—are on the same place: their breast. She once invites Ritsuka to a coffee shop, and reminisces the first time she and Kouya met. When she saw what Kouya did to her wrists, she got the cutter and slit her wrists too, in which point Kouya apprehends her. She replies that whatever injury Kouya has, it will also appear on her. She holds her wounded hand to her heart, and says "it's beginning to hurt, here." In the coffee shop, Ritsuka accidentally spilled some coffee on her blouse, a result of being surprised when she said she's going to show him her mark (her breast). She goes in the bathroom claiming it is in order to remove the stain since it is going to spread. Inside, she opens her blouse and stares at her mark, which is beginning to fade, meaning it won't be long before she is not going to be Kouya's sacrifice anymore.
During their fight with Loveless and Soubi, her mark has completely disappeared, so she had to endure all of Soubi's attack. Kouya doesn't know so she proceeds fighting. However, when Yamato is brought on her knees by the pain, Kouya begins suspecting, in which Yamato vehemently refuses and says "it doesn't hurt!" Kouya forcibly opens Yamato's blouse, and sees that the zero mark has completely disappeared. She withdraws from the fight, even if it means certain outcomes. They decide to die together and be reborn than to live separately.
Ritsu Minami (南律, Minami Ritsu)
Voiced by: Takehito Koyasu (Japanese); Marc Matney (English)
He is Soubi's former teacher, and held a very influential role in Soubi's life when he was still his teacher. The extent of their master/slave relationship is left unknown, but it is clear that Ritsu misses those days. In the few times they have made contact within the current timeline, Soubi acts very brusque and treats him with disgust. It is also revealed that he was the one who took away Soubi's virginity. Ritsu is said to have been the Sacrifice to Soubi's mother, which led to him and Soubi meeting.
In one of the Zero-sum chapters Ritsu has his eyes gouged out by Seimei, so he could write a message to Ritsuka on the wall in the man's blood.
In the latest chapter of Zero-Sum Monthly Comic (March 2008), the seven members of Septimal Moon were revealed. Included as members were Ritsu Minami, Nagisa Sagan, and Nana. Both Ritsu and Nagisa had previously denied any affiliation with the Septimal Moon.
Ritsu is the only blind character in the series.