Grendel Jinx is the daughter of the head of The Council Of The Unseen and a senior agent. With both her parents busy, she was generally lonely and mostly friendless during her school years. It also didn't help that she turned to her enemy for friendship, even if said friendship is vastly skewed in her favour.
Grendel is a megalomaniacal, kleptomaniac, cake-mix loving agent, who, until The Council was destroyed by the protagonist, was second in command, although she generally spent less time on doing Council work, and more on annoying the protagonist by either getting him into trouble or trying to steal everything he has - all in the name of trying to get him to work for The Council.
Despite the constant fights between her and the protagonist, she does genuinely like him, although she does keep it hidden.