Michishio family
Nagasumi's Father (Nagasumi no Chichi)
Nagasumi's father is always shown as an object of suffering, due to his wife's dying love for him. Whenever his wife shows her interest in Masa, he is shown in a depressed state, like the time she offers Masa her home cooked meal, he is shown coughing on a burger and then subsequently harangued by his boss to get back to work.
Nagasumi's Mother (Nagasumi no Haha)
Nagasumi's mother is the easy going type, and has an interest for Masa, which her husband is not particularly happy about. Current events in her life have caused her love towards her husband to gradually die out.
Nagasumi's Grandmother (Nagasumi no Sobo)
She makes an appearance early in the story and allows Nagasumi to stay at her house during their stay with her.
Seto Family
Gōzaburō Seto (Seto Gòzaburò)
Gōzaburō is the boss of the Seto Group. He dotes on his daughter and hates Nagasumi for, as he sees it, trying to steal her away. By pulling strings at the ministry of education, Gōzaburō, together with many of the Seto Group members, enrolls in the staff of Nagasumi's junior-high school in order to watch over Sun, thus becoming Nagasumi and Sun's homeroom teacher. Even though he is the Yakuza head of the Seto group as well as a powerfully built man with a formidable attitude, he has an overwhelming fear of cats (or kittens) due to an incident that happened in his childhood (which parodies that of Doraemon's fear of mice, both in its origins and his reaction when seeing the object of his phobia). He fears his wife as she will not hesitate to hand out punishment
Ren Seto (Seto Ren)
Ren is Sun's mother. She likes to encourage Sun and Nagasumi, The most feared member of the Seto Group she usually tries to keep her husband and any other Seto Group members in check, often handing out extreme forms of punishment if they step out of line and has the final say on all matters. She also enjoys teasing Nagasumi, whom she thinks to be cute. She transfers with several other members of the Seto Group to Nagasumi's school, becoming the new school nurse. Like her daughter Sun When Ren enters chivalry mode pink flower petals can be seen in the background. Ren on appearance is seen as a kind and gentle person but shows behaviour much like a Yakuza member. Her name is likely a reference to the mythical creature, the Siren.
Seto Group Members
Masa is a member of the Seto Group. He has an afro and always wearing sunglasses. Masa is the one who, through performing CPR, "stole" Nagasumi's first kiss. He also transfers to Nagasumi's school, becoming his math teacher (whose word problems tend to describe to the mathematical aspects of underworld gang operations). There is a running gag of Nagasumi displaying an attraction towards Masa since their 'first kiss' in the first episode. Masa is the only one apart from Sun's mother who doesn't object to the engagement and sometimes even gives advice to Nagasumi. Usually he is portrayed a silent man. However, when he drinks he tends to talk too much and reminisce. He is revealed later in the series to be the long-lost brother of Shiranui Akeno. Masa cares a lot for his sister Shiranui Akeno and is very protective of her often beating up those who are aggressive towards her. Masa is one of the few Merfolk not afraid of cats, explaining that he decided to overcome the fear as a child.
Shark is a member of Seto Group initially summoned by Sun's father during the Michishio family's initial visit to the Seto family home to attack Nagasumi. In human form, he later becomes Nagasumi's P.E. teacher. By his thinking, the sooner Nagasumi dies, the sooner everyone can return home. He often acts on this logic, trying to eat him. According to him, the answer to all problems is by eating the person causing the chaos, be it Sun's or Lunar's fans or a suicide bomber from his own group. he often seen sleeping and unlike Kai Mikawa he has the ability to only transform his head into that of a shark while his legs remain human.
Nakajima is another member of the Seto Group and is actually an octopus. He too becomes a teacher at Nagasumi's school. Unlike the others, his human form is never shown in the entire anime. Most of the time he appears stuck inside the school due to his huge size. It's also shown that some of his tentacles are missing. Some scenes taking place in the cafeteria will show octopus being fed to the students.
Edomae Family
Lunar's Papa (Runa-Papa)
Never referred to as anything but Lunar's Papa, he is Lunar's father and leader of the Edomae group (in which whereas the Seto group's holdings are in business, the Edomae group focuses in construction and real estate). He is two meters tall and constantly dressed in a black skintight suit. Lunar's Papa is a man of very few words and is modeled after the Terminator. He is an extremely well built man (though in the manga he looks more thin), a point of humor in the show due to his body sounding like metal when hit. His actions are often direct references to the Terminator movies, such as sinking into an incinerator giving a thumbs up while even saying in English "I'll be back". His appearance theme is a parody of the lead music score from the Terminator movie 'Judgment Day'. Despite his cold and unemotional appearance, he cares very much for his daughter. He does everything he can for her, though discretely so. He is also extremely rich, as evidenced in one episode of using 10 billion dollars US (in cash) to keep the production of a film Lunar is acting in going. In the final episode, he is revealed to have laser eye beams. He is often seen in female sailor’s uniform which Lunar hates, he also has influence in the ministry of education able to get into Nagasumi's school pretending to be a middle school student so he could get closer to Lunar.
Amano Sagami (Sagami Amano)
Lunar's mother and Lunar's Papa's ex-wife. She left the two when Lunar was five years old.
Others
- Class President (Class Rep in the English Dub)