He is not seen, and you cannot re-enter the portal to the other galaxy. Presumably he is trapped there, and only time will tell if he ever returns to menace Arturus.
With Mordred
You can't costumize him.
I think that it was Mordred.
It is next to the Mordred Museum at the entrance to the island.
In the left corner of Mordred's lair.
Mordred does not appear as an interactive character, so it is apprently not possible to copy him.
Defeating Mordred and his warrior suit wins the island, and you receive the Island Medallion and 50 Poptropica credits.
go to astro nights and go to the castle and find your rocket on it. Fly back to mordred
When you defeat Mordred, the game is over, and the Princess will kayo him for you. (see the related question)
Twice.
There are two books in the library and one in Mordred's laboratory.
The character named Mordred is from the legends of King Arthur, as are most of the references in Astro Knights island. His story in the "real-life" historical annals is quite different from the one in the Poptropica game.