What is the name of the retired star who rules the Dufflepuds?

The retired star who rules the Dufflepuds is Coriakin.

Coriakin appears in C. S. Lewis’s The Voyage of the Dawn Treader, the third-published Narnia novel, released in 1952. He was once a star, but Aslan assigned him a period of penance: he must govern the Duffers and guide them toward wisdom. The books never explain exactly what wrongdoing caused his punishment.

Coriakin lives in a large house on an island and is surrounded by magical objects, including a spellbook and a Bearded Glass. His subjects resist his attempts to make them wiser, so he transforms them into one-legged creatures called Monopods. They protest that they have been “uglified,” then secretly make themselves invisible.

Lucy eventually breaks the spells affecting the island. She discovers that Coriakin is not the frightening tyrant the Dufflepuds imagine, but a kind, humorous and patient guardian. A common mix-up is Ramandu: he is another star and explains Coriakin’s past, but he does not rule the Dufflepuds.