Sill Plain is a human female with pink hair, green eyes, and standing 156 cm tall. Purchased as a slave by the adventurer Rance for 15,000 gold before his earliest adventures, she works for him performing domestic duties such as cooking, laundry, and shopping, while also providing combat support. Though she often refers to herself as his assistant, her initial enforced servitude via "Absolute Obedience Magic" has worn off; her loyalty now stems from romantic feelings for Rance.

Originally from a wealthy noble family in Zeth, circumstances led to her being sold into slavery young. Her noble education granted her extensive knowledge of history, culture, and magical creatures. She is proficient in offensive and healing magic, specializing in barrier creation and the targeted damage spell "Magic Missile."

Sill's personality blends kindness, loyalty, and timidity, frequently appearing clumsy, naive, or airheaded. She remains a dedicated hard worker. Despite enduring teasing, domestic violence, kidnapping, and confinement due to her association with Rance, she stays protective of him. Her devotion includes wishing for lifelong companionship and experiencing distress during separations. She fears his potential descent into villainy, as when he led a bandit group. Her capture by the Helmanian Empire prompted Rance's rescue campaign.

Across adventures, she participates in missions from rescuing missing persons to confronting demonic threats and political conflicts. Her skills prove valuable in diverse scenarios, like investigating disappearances or supporting military operations. During a period apart from Rance, she temporarily adopted the superhero identity "Pink Mask" to aid others. In collaborations, such as with a flower knight order, she assists in combat and domestic duties, openly acknowledging her slave status while emphasizing her voluntary care for Rance. A curse related to her enslavement exists, though its specifics remain undefined.