Ambrosius is an Abyssinian longhaired cat, living into an house in the forest. He's an important character in the second book, collaborating with Francis during part of the investigations.
He's characterized by intelligence and writing skills, but also by a constant babble.
(SPOILER): At real Ambrosius is attempting to obstruct the investigation of Francis (staging as a fake Hugo, a black cat blamed for the murders) he's aware of the true identity of the killers and he's partly responsible for the killings. The wild she-cats were instigated by Ambrosius's words and even if it wasn't planned by him, the state of despair and awareness of their doom forced the wild felines to commit the murders.
The end of Ambrosius is ironic, because he's murdered by the wild cats before their departure to not leave someone who knew the truth behind them alive.