Sella is a homunculus created through Einzbern alchemy. Initially deemed a failure in the Holy Grail project and marked for disposal, she was instead repurposed as Illyasviel von Einzbern's maid and magic instructor. She exhibits the common Einzbern homunculus traits: long pale hair, crimson eyes, and a mature appearance despite being chronologically around two years old. Typically seen in a maid uniform, she is also depicted in casual attire. Her personality is strict, formal, and deeply protective of Illya, leading to suspicion of outsiders like Shirou Emiya. Beneath her stern exterior lie insecurities about her body and a fondness for nature, preferring plants and animals to humans. She possesses superior magic circuits and tutors Illya in sorcery, but her fragile homunculus body makes her unfit for combat. Her duties include maintaining the Einzbern castle's courtyard and gardens, where she cultivates flowers symbolizing hope for Illya. During the Fifth Holy Grail War, she accompanies Illya to Fuyuki alongside Leysritt. In the *Unlimited Blade Works* route, her absolute priority is Illya's safety, culminating in her death defending Illya from Gilgamesh.

In the *Fate/kaleid liner Prisma Illya* universe, an alternate timeline without the Fourth Holy Grail War, Sella's role changes significantly. Irisviel and Kiritsugu Emiya bring her and Leysritt to Fuyuki to care for Illya and Shirou during the parents' absences abroad. Functionally part of the household, Sella assumes primary responsibility for domestic management, including cooking, cleaning, and childcare. Her matronly demeanor and authoritative role sometimes lead visitors to mistake her for Illya's mother. While retaining seriousness and a strong work ethic, her personality becomes more relaxed and expressive. She exhibits overt frustration toward Shirou, especially regarding his intrusion into "her domain" like the kitchen, viewing his culinary skills as a challenge to her responsibilities; this rivalry is non-malicious, stemming from her dedication to household duties. She displays heightened protectiveness over Illya's routine and well-being, frequently chastising Shirou for perceived neglect of his brotherly duties and Leysritt for her laziness. Her relationship with Leysritt resembles a sibling dynamic: Sella is disciplined and meticulous, contrasting with Leysritt's lethargy and indifference to chores, creating friction alongside underlying loyalty. Sella exhibits humorous vulnerabilities, including sensitivity about her modest bust size and weight, subjects of occasional teasing by Leysritt and Illya. She remains unaware of Illya's activities as a magical girl. Her abilities in this universe emphasize domestic proficiency—nursing, cooking, home management—though sparring with Leysritt implies combat capability.

Her portrayal remains consistent as a dedicated, occasionally rigid, guardian figure throughout *Prisma Illya*'s seasons, movies, and OVAs.