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. (Source: myanimelist)