Akeno Shiranui  

Akeno-chi, 不知火明乃

Akeno is not just a mermaid, but she's also responsible for examining other merpeople to ensure they meet certain criteria before allowing them to remain in human society. This includes both resembling humans and avoiding any suspicion of their true identities. Initially, Akeno had a hostile demeanor towards the main characters. However, as time went on, she developed feelings of compassion for them and saw her duty as a Knight-in-training as an excuse used by the final antagonist.