A member of the Seto Group. Masa is the one who stole Nagasumi's first kiss (he was actually performing CPR, but anyone would use that as an excuse to say they got a kiss from a man as beautiful as him). Nagasumi's mother and father are very attracted to Masa, but he often shoots their love down for he only loves his close friend Mark. He also transfers to Nagasumi's school, becoming his math teacher. He is often seen with his close friend Mark who he is often compared to in terms of beauty.

Michishio Nagasumi's life couldn't be any more normal. In an odd twist of events, during his summer vacation he ends up almost drowning in the sea. Luckily, the cute mermaid Seto San appears to save him. However, San is from a yakuza mermaid family and according to their law, if a human is to catch sight of a mermaid, either he or the mermaid must die. The only other way is for Nagasumi to be taken in as a family member, marrying San. In attempts to save both of their lives, Nagasumi asks for San's hand in marriage. Nagasumi's summer vacation reaped more then what he would expect, as he must now protect San from others finding her secret out. 

Christopher Robin Sabat is an American voice actor, producer, ADR director, and line producer at Funimation. He provided voices for a number of English versions of Japanese anime series such as a number of Dragon Ball characters, including Vegeta, Piccolo, Yamcha, and some others. In Funimation's re-dubbing of One Piece, he voices Roronoa Zoro. Other major roles include Kazuma Kuwabara in Yu Yu Hakusho, Alex Louis Armstrong in Fullmetal Alchemist, Tatsumi Saiga in Speed Grapher, Kurogane in Tsubasa Reservoir Chronicle, Daisuke Jigen in Lupin the Third and All Might in My Hero Academia. In video games, he voiced Garland in Dissidia: Final Fantasy, Rundas in Metroid Prime 3: Corruption, Alex D in Deus Ex: Invisible War, and both Captain Smiley and Star in Comic Jumper. He is the founder and director of OkraTron 5000, an audio production company that provides support for some of Funimation's and other dubbing titles.

The OVAs are new stand alone shorts with characters from the series and introduces two new ones. It is now the third year of school, San and Luna's dads are still being over protective. All the while Nagasumi has his hands full trying to keep everyone around him under control and rescuing damsels in distress. 
(Source: Anime News Network)

A young boy gets saved from drowning by a mermaid but, according to mermaid law, if a human sees a mermaid's true form, both have to be killed. The only solution to the problem is for him to marry her. But can they keep her secret from being found out? (Source: Animewaves)

An exclusive picture drama released with the Blu-Ray Box Set of Seto no Hanayome. The first part starts out with gags and the characters drawn in a simplistic chibi style. It's followed by a quick recap and then goes into the main picture drama story. 
 
(Source: Kitsu).