At the giant asteroid (X-11 Y-80), pull out the Golden Key to enter the portal. Solve the door puzzle to enter the castle, then battle Mordred as the owl Merlin and as your character.
The Door Solution
Press each of these 10 panels once : all four panels in the 2nd row, the middle two panels in the 3rd row, and all four panels in the 4th row.
(To beat Mordred, see the related question.)
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.
Twice.
There are two books in the library and one in Mordred's laboratory.
When you defeat Mordred, the game is over, and the Princess will kayo him for you. (see the related question)
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.
Actually, Merlin has little to do on Astro Knights Island. He recovers the fuel rod from the dungeon when the robot self-destructs. And he battles Mordred at the very end of the island.