"Spiral Shell" Maki is an assassin working for the Seto Group, as well a devoted friend of Sun. She, like G�zabur� Seto, dislikes the idea of "losing" Sun to Nagasumi, and will often distract Sun with sweet talk in order to fire her shell gun at Nagasumi. She also thinks that Nagasumi and Luna should be together. (Source: Wikipedia)
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 he can transform in only a sharkhead, but also a whole shark as seen when he's in the sauna. ( partly wikipedia )
Saru's little sister who appears in the original manga and the OVA series. After being saved from a burning movie theatre by Nagasumi, she calls him "Nagasumi Willis" after "Bunta Willis" an action hero that parodies Bruce Willis and Bunta Sugawara. (Source: Wikipedia)
Saturn appears in Seto no Hanayome OVA as an old friend of Seto San who pays a visit to the school.
Hideyoshi Sarutobi, often referred to as Saru, is a friend of Nagasumi who is often portrayed as a perverted monkey (a pun on his name; "saru" meaning monkey). He later became Kai's "servant" and always protects and helps him when he is in need (although reasons are more likely because of his fortune). Saru occasionally appears dressed as an old hermit, "ero-sennin" (erotic/perverted thousand-year-old), who periodically appears to offer insights of perverted sagacity. (Source: Wikipedia)
Akeno is not only a mermaid but also a "mermaid examiner". She has the responsibility of deporting all the mer-people who fail to qualify as not only resembling humans but also not arousing any suspicion of their true identities. When first introduced she had a hostile attitude towards the other major protagonists. This later changes as develops feelings of compassion towards them and also as she finds her "duty" as a Knight-in-training as an excuse used by the final antagonist. It is also revealed that she is trying to break up the young couple. It is also revealed that she is in-fact Masa's younger sister. Her name is related to Shiranui Sea and Venus (明��明星, ake no my�jy�) Venus in the eastern morning sky. In the OVA series, it is shown that she started to develop feelings for Nagasumi. [wiki]
Nagasumi is the protagonist of the story. Nagasumi is likely the only one in his family who has a clear head, apart from his grandmother. When push comes to shove, Nagasumi demonstrates large inner strength. (Source: Wikipedia)