Nenneke is a priestess of Melitele, and the indisputable head of the Temple of Melitele in Ellander. She's known Geralt since he was young, treats Dandelion with fond contempt and has met Yennefer in several occasions.
The priestess is a motherly, stern figure, but she scoffs at being called Mother. She is more or less a mentor to Iola, and Nenneke's pharmacy is renowned for its potions, elixirs, salves and ointments, some of which require herbs growing only in the crystal-roofed greenhouse on the temple grounds. The wife of duke Hereward, Ermellia, is known to enjoy aphrodisiacs Nenneke cooks up.
When the duke and the Order of the White Rose wanted Geralt to leave Ellander, she defended him, saying that the duke had no jurisdiction over the temple and that the witcher could stay there for as long as he wanted.
Unlike in the books, Geralt apparently met Nenneke for the first time after his fight with the striga. While he was recovering from his wounds, he grew delirious and mistook Nenneke for his mother. With fondness, Nenneke accepted the title and the role of a mother-substitute for the witcher.
Later, Nenneke took a stand against the policies enforced by the Order of the White Rose, insisting the temple she led was open for everyone, and taking in refugees from Cintra, including Ciri, and elves.
Nenneke was killed, together with all the other priestesses, in stark contrast to the books, where she survived the saga.