Sella is a homunculus created by the Einzbern family to serve as a maid and to teach Illya sorcery. Despite her appearance, she is only about two years old.

Sella is highly protective of Illya, whom she loves and cares for deeply, and thus is very suspicious of Shirou Emiya. Despite her apparent coldness to everyone but Illya, Sella is actually rather fickle and insecure, particularly about her body, and Leysritt and Illya often tease Sella for her harsh personality.