Sella serves as a caretaker within the Einzbern household where Illyasviel von Einzbern lives an ordinary elementary school life. Alongside Leysritt, she watches over Illya and her adopted brother Shirou Emiya while their parents, Irisviel and Kiritsugu, are overseas. She and Leysritt came to Fuyuki City from the fallen Einzbern family after the Fourth Holy Grail War was prevented, enabling Illya's normal life free from Grail War rituals.

Created by the Einzbern family as a homunculus, Sella is approximately two years old despite her mature appearance. Her primary duties involve managing household affairs and guarding Illya and Shirou. She skillfully handles domestic tasks like cleaning, nursing, and cooking. While her combat abilities are not detailed, she regularly engages in hand-to-hand sparring sessions with Leysritt.

Sella takes her responsibilities seriously, often maintaining formalities such as wearing her original maid outfit during Illya's friend visits. She displays a stern demeanor towards Shirou, emphasizing his role as the eldest son and urging him to uphold household duties. Her interactions with him include reservations, though these manifest comically. Flashbacks show she historically wore her hair in a mid-length ponytail during Illya's younger years.

Within Fate/kaleid liner PRISMA☆ILLYA 2wei Herz!, Sella appears in the special episode "Beast, Futatabi!", featuring comedic scenarios with Illya, Miyu, Chloe, Leysritt, Shirou, and the Kaleidosticks Ruby and Sapphire, reinforcing her stable caretaker role amid surrounding magical conflicts.