Morgan is a Cornish piebald rat with a stump for a tail and pierced ears, who is Jupiter's lieutenant in the first book, The Dark Portal. He is selfish and cold hearted, much like most if not all of the other sewer rats who worship Jupiter. He keeps an old rag tied around his stump of a tail. He was thought to have drowned in the sewers when Jupiter's great tail knocked him into the water at the end of the story during the fight between the mice and Jupiter. He is the only rat who knew fully of Jupiter's plans to release the Black Death once more, by making the sewer rats dig large mines until they dug straight into a pit used to throw plague victims into. It is also revealed that Morgan killed Jupiter's previous lieutenant in order to take his position.

Confirming he did not die in The Dark Portal, Morgan appears in The Final Reckoning leading an army of city rats who know him as "Old Stumpo." The rat army kills Piccadilly's family and friends in Holeborn. After being contacted by Jupiter, he is seen throughout the story leading his army to Jupiter's base. His last appearance in the story is when he fights Piccadilly in the snow. He later commits suicide. He makes a short cameo in the third Deptford Histories book, Thomas.