Sella is an Einzbern homunculus, created through alchemy for the Holy Grail ritual but deemed a failure and marked for disposal. Instead, she became Illyasviel von Einzbern's caretaker and magic instructor. In the *Prisma Illya* timeline, she and Leysritt manage the Emiya household in Fuyuki City during Irisviel and Kiritsugu's absence. Her duties encompass household management, teaching Illya sorcery, and tending to the Einzbern castle's courtyard flora and fauna.

Chronologically about two years old, Sella appears as a young woman bearing the Einzbern homunculus traits of long pale hair and crimson eyes. She typically wears a formal maid outfit for Illya's social engagements, switching to casual wear privately. Her personality blends stern professionalism with underlying insecurity; she enforces strict household rules and is protective of Illya, yet shows vulnerability about her physical form, often teased by Illya and Leysritt for her rigidity. She remains suspicious of Shirou Emiya, viewing him as irresponsible, but expresses warmth through her dedication to gardening and animal care.

Magically, Sella excels as a teacher and in household magecraft, possessing superb magic circuits for instructing advanced sorcery and the ability to render herself inconspicuous to non-magi. Her fragile homunculus body grants her no combat aptitude. She also holds practical skills including cooking, nursing, cleaning, and licensed driving.

In *Prisma Illya 2wei!*, Sella's role remains focused on domestic management and supporting Illya's daily life, without direct involvement in the season's primary conflicts involving Class Cards or Kuro.