Nakajima is a member of Seto Group.  As a very minor supporting character, not much is known about him, except his human transformation is weaker than the others, as can be seen in numerous scenes where he accidentally turns back into his octopus form.

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. 

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? <br><bR>(Source: Animewaves)

Nakajima Octopus