Saffron is a large Chartusian old tomcat, leader of the Sewer's cats community met by Francis in the second book.
He had a terrible childhood, being starved by his human master (Saffron's brother died of thirst) and tortured to accomplish the man's fetish and "artistic" desires including the implantation of visible earrings. Saffron, after being burned by his master, managed to jump into a toilet's water to ease the pain and was inadvertently pushed into the sewer where he joined the community of cats until leading them. As all the other cats of the community, he turned blind after some time.
When Francis entered their territory, Saffron orders Rhodes (his best warrior and killer) to perform an execution of the intruder: his intent is to prevent the possibility of human's discovery about his community (if many cats start to visit the sewer, humans could notice the unusual feline presence underground and decide to destroy it).
(SPOILER): After the death of Rhodes, Saffron realizes the identity of Francis (having heard of his skill as detective) and he's more friendly with him, telling his story and the community's mores.
He and Niger asks Francis to help them: he has to track the suspected killer of a number cats whos bodies that are being found in the sewers, carried by the water. Saffron believs Hugo to be responsable of the killings: a former member of the Sewer's cats who left.