She is Kenji's younger blue-haired sister. Being gentle, being active, innocent character. 

Lamune tells the story of a boy named Kenji and his childhood friend Nanami. Although, everyone seems to know that they have a relationship, neither are determined to push it anymore than close friends. The story goes through flashbacks, explaining why they are such close friends and any action doesn't seem to push them apart, but draw them closer.
 
(Source: Anime News Network) Note: Later rebroadcasts included short additional scenes that were first released in the DVD volumes.

Suzuka Tomosaka