Sella is Illya's maid.
Sella was a defective Homunculus of the Einzbern family in their goal of creating the Holy Grail. She was given new life as a maid to Illyasviel. Unlike Illya and Leysritt, Sella has no special properties to her creation as she was created as a normal Homunculus.
Sella serves as a minor character within the Fate/ Stay Night franchise. She was originally going to be designed as a male butler who served Illya. However her current design was later utilized due to insistence from one of the visual novel's art directors. She makes appearances in Ufotable's TV adaptation of Fate/Stay Night Unlimited Blade Works, Carnival Phantasm and seasons of the Fate/Kaleid Line Prisma Illya anime franchise. She is voiced by Terada Haruhi in Japanese and Beth Lezarou in English.
Sella stands at 163 centimeters (about '5'4") with her three sizes being 75 centimeters for bust, 58 centimeters for waist and 79 centimeters for hips. She has long silver hair and red eyes. In the Fate/ Stay Night series, she is normally dressed in her maid attire. In Fate/ Kaleid, she wears a long-sleeve yellow shirt with a brown apron over it and a long brown skirt.
Sella is very protective of Illya, to the point where she has distrust of Shiro. Despite being seemingly cold on the surface, Sella does have emotions as she isn't hesitant at showing her distrust toward Shiro, is rather self-conscious about her body and often finds herself getting teased by Illya and Leysritt for her cold personality.
In Fate/ Kaleid, Sella retains her serious personality from the Stay Night series as she often finds herself irritated with the often bizarre predicaments that take place in the household.
Sella is sent along with Leysritt to serve as maids and guardians to Illya throughout her participation in the Fifth Holy Grail War, with her role being to serve as a maid and magic teacher to Illya. She mostly served as a background character in all routes of the series and helped dress Illya in the Dress of Heaven following Leysritt's sacrifice.
Sella retains her role as serving as one of Illya's maids. However, her presence in the series here is to serve as the "normal person" within the household as she finds herself often exasperated with the eccentric personalities and sometimes bizarre activities that take place within the Einzbern residence.
For Fate/Stay Night, Sella's body is not useful for combative scenarios as she lacks special properties for it like Leysritt and Illya, despite having superb magic circuits. Despite this, she was still an excellent magic teacher for Illya.