Maria is the supervising teacher of the Neighbors Club; she is a ten-year old nun. While not unintelligent, she is often rude and immature and refers to others as "Rotten mandarin." She develops a sibling-like relationship with Kodaka who regularly makes her packed lunches, and is affectionate with him. She is also gullible, which Yozora uses to her advantage.
It was later on revealed that she was never a teacher nor a nun at the school, which almost causes the club's disbandment if it weren't for Sena's intervention, who called up her father to appoint her as a special part-time teacher.
(Source: Wikipedia, edited)