Sayo Aisaka is student number one in the class, though she is actually a ghost that no one can initially see. She died in 65 years before the series, and she has been unable to leave the school campus or interact with anyone. She still goes to the same class, and sits in the same seat, which always remains empty. She desperately attempts to make friends, though she always ends up being treated as an evil spirit. While trying to scare people as a ghost, she fails miserably and often trips, despite not having feet. After Negi starts teaching the class, she is eventually noticed by him over time.
Source from Wikipeida