Akeno Shiranui

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 (明けの明星, ake no myōjyō) Venus in the eastern morning sky. In the OVA series, it is shown that she started to develop feelings for Nagasumi. [wiki]