In Greek mythology, Demeter is the goddess of the harvest who governed over the fertility of the land and the seasons. When Hades abducted and married her daughter, Persephone, Demeter plunged the world into an almost endless winter, forcing Zeus to mediate between her and Hades. As a result, Persephone was allowed to leave the underworld for a certain amount of time. The winter months of the year occur when Persephone returns to Hades.
Demeter restores the protagonist in the Womb of Grief following his mortal wounding at Alex's hands, calling him by the nickname "My Harvest" and imploring him to grow into strong and healthy wheat. In exchange for his resurrection, she asks that he aid her in collecting fruits from the Mothers and Wardens of the Womb.
She lacks physical power, admitting to being weaker than a pixie, but makes up for it with her powerful healing capabilities and non-combat magic that can hide her and the protagonist from others. Demeter is on friendly terms with the mothers (Anahita and Ishtar being the first) who are both prisoners and protectors of the fruits, and is set on her course no matter what happens as she reveals with her talk to Maria. Should Alex die near the end of the original routes with Amon's fruit she will lament the rotten state of it, cursing humanity for destroying it and hoping they all die horribly on Neutral and Chaos. On Law she is saddened, but supports Zelenin and the protagonist for their path before vanishing.
She reveals her true nature on the New/+ routes, where she takes the fruit away from the protagonist and reveals her true role as a servant to The Three Wise Men alongside fellow servant Mastema. She laughs at the fate that shall befall them on Chaos/Neutral+ and the killing of Mem Aleph or offering her sincerest gratitude in Law+, before leaving for the Empyrean ascent on all three. Her true motives unveiled by Louisa Ferre.
Demeter appears during the final battle as Shekinah summons her, telling her to offer her life for a world of Law. Fearing her own death and inability to bring fertility to the world under the God of Law, she reluctantly complies and uses her full healing powers, her life withering away as she laments and dies. Her healing power now bestowed upon Shekinah as her essence is absorbed.
Demeter portrays herself as childish and innocent to hide her true nature, a ruthless agent of Law who has to rely on others due to her lack of power, even having a sadistic streak and amusement at the actions of others. Her loyalty to Law and desire to bring fertility is genuine. Demeters willingness to serve means she was even willing to sacrifice her brother Zeus using the protagonist, and eventually, her own life, to make the world of Law a reality.
Demeter can only be unlocked for fusion on a New Game Plus, after the player has seen one of the new endings in Redux. Her special fusion requires Quetzalcoatl, Mayahuel and Anahita.