Nagisa is a young boy who has a crush on Hanon. Ever since retrieving a music sheet for Hanon, he has been trying to woo her. Unfortunately for him, the sheet music is of the tune Tarou Mitsuki dedicated to Hanon, which only reminds her of her fixation on the much more refined and older man. 

In the manga, he says that he is two years younger than Hanon, which in addition to her still being fixated on Tarou, makes things more complicated for Nagisa. 

~!In the end, Hanon falls in love with Nagisa and they start dating.!~

After the final battle against Gaito, Sara had given her life to remain with him. The sea world is now at peace, but only with six mermaid princesses. 
While Caren, Noel, and Coco have returned to their kingdoms, it should be time for Lucia, Hanon, and Rina to do the same. After all, Mitsuki Tarou is leaving for Germany, and Doumoto Kaito is heading to Hawaii. Lucia is devestated, and when things couldn't get any worse, Kaito is lost at sea! Worst of all, a strange angel named Michel has appeared, wanting the mermaids to join him. But on the lighter side, Sara's spirit tells Lucia of Sara's successor, a young mermaid named Seira. The mermaids are in for another adventure!

Lucia is the new girl at school. She and her sister run a public bath that's all the rage. When Lucia meets a terrific-looking surfer boy, there's just one little problem: Lucia is a mermaid-not just any mermaid, but a princess on an important mission to save the seven seas from an evil force bent on taking control of the marine world. Such a responsibility doesn't leave much time for romance. But Lucia vows to protect her world and win the heart of handsome Kaito. 
(Source: Del Rey) 
Note: Includes the one-shots Moonlight Goddess Diana (MOONLIGHT GODDESS ディアナ), Nude ni Natte (ヌードになって, Get Nude) and Cherry Blossom (チェリー ブロッサム), contained within Volume 7 of the manga.

Nagisa Shirai