Nico is a fairy who takes on the form of a cat. He accompanies Lydia on her journey with Edgar.
While he can walk on four legs like a normal cat, he prefers to walk on two legs like a human. He is able to turn invisible, and can read and write human language. Nico was Lydia's mother's companion, as well.
He likes liquor, and fine tea. He is also very particular about being treated like a gentleman rather than a cat, and as such obsesses over his attire (most frequently a single neck ribbon, but he has been known to wear suits and tailcoats), and the state of his fur and whiskers. On a few occasions, Edgar has provoked him deliberately by scratching Nico and petting him like a mere cat.
He is rather cowardly, often disappearing when things turn nasty. Despite his inability/unwillingness to engage in physical combat, his assistance is invaluable to Lydia. He often gathers information for Lydia from the fairies in the area, as well as acts as a guide when Lydia needs to travel to or from the fairy world. He is also something of an adviser to Lydia.
In recent times, Edgar has taken to bribing Nico with things like fine food or a gentleman's "essentials" (a cane, a tailcoat, letter-writing set, etc), in order to find out developments around Lydia.

(Source: Wikipedia)