Munkustrap was first mentioned in T. S. Eliots poem 'The Naming of Cats'. When Andrew Lloyd Webber came to do the show based on a group of poems by Eliot, Munkustrap was made to be a strong, protective silver tabby.
Munkustrap is the protector of the tribe and is thought to be second in command after Old Deuteronomy, who is the tribe's wise and highly respected leader. He is thought to be a relation to Old Deuteronomy, some theories being that he may be his son or even his grandson. He is also thought to be brothers with the Rum Tum Tugger and Macavity.
Munkustrap is the narrator of the show and in some parts does not join in the dancing as much as everyone else. He likes to keep all the kittens in line and is quite a serious character. He is usually one of the tallest Toms in the show.
Micheal Gruber, who played Munkustrap in the video, had this to say about his character -
“Munkustrap is really the storyteller – he's entrusted with giving the information to the other, maybe younger Cats who have not been to the Jellicle Ball before. He gets up and says exactly what they’re here to do, he's also caretaker of the kittens, wants to make sure everybody’s safe, in all the Macavity scares he’s there, to – protect the trible. So he is the protector."
(Image from Photobucket, Interview information from Felina's Jellicle World)