Seira Mimori is a classmate of Meimi Haneoka, and is the only person who knows of her escapades as Saint Tail. Seira is a nun-in-training who is usually found at the chapel after school where many people go to her and relay their personal problems, usually problems which involve something being stolen. In turn, she relays this information to Meimi, who helps this person out as Saint Tail.

Seira is rather quiet and contempative, the opposite of her best friend Meimi. She will sometimes stand in for Meimi when she needs a double during the series. Seira is also very observant, and caring, expressing her concern when she is frightened that Meimi will be discovered.