"I am the dark thought in their waking hours, I invade their dreams and bring horror! I am the essence of night, the terror around the corner, the echo behind!"Jupiter is the main antagonist of The Deptford Mice trilogy.
Born as Leech, he was the son of Imelza and brother to Dab and the real Jupiter. As a kitten Leech was jealous of his brother, whom Dr. Spittle had chosen as his familiar. Dr. Spittle constantly criticized and abused Leech because he was the ugly runt of Imelza's litter.
After Dr. Spittle had imprisoned the spirit of Magnus Augustus Zachaire in a bottle, Leech learned the magic arts from him, despite the fact that only Jupiter could use it at the time.
But finally, when Dr. Spittle's apothecary shop was burning down, Leech tossed Jupiter down a burning chimney and the cat fell to his death. After this, Leech took Jupiter's name. Becket led the badly-burned cat down into the sewers, where he crawled into the dark portal and (because he drank from the elixir of life) he was able to remain there for centuries, worshipped by rats.
However, when Audrey Brown confronted the evil cat and threw her mousebrass at him, he fell into and drowned in the sewer water. When his body washed up on the shore near the River Thames, it was tossed into a fire and burned.
After this, his spirit possessed a tattoo on the fortune-teller Madame Akkikuyu's ear, claiming that he was Nicodemus, a spirit trapped in limbo. When the rat agreed to help him, it was revealed that Jupiter wanted to take over her body since his was destroyed. Akkikuyu tossed herself on the fire before this could happen, though.
Unfortunately, this enabled the spirit of Jupiter to return. When Alison Sedge broke Akkikuyu's crystal ball, Jupiter was free. Wanting revenge, he smothered the world in an eternal winter. He stole the Starwife's Starglass, and used this to remove the stars from the sky. He planned to remove the sun the next night, but before he could do anything else, however, Audrey confronted him once more. She threw a snowdrop flower into his mouth, and this condemned him to the doom of life throughout eternity.