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

Like Ilya, she has silky blonde hair and deep crimson eyes. Sella is rather fickle and insecure (especially regarding her body), and is very suspicious of Shiro Emiya.

(Source: Type Moon Wikia, Wikipedia)