You can't costumize him.
With Mordred
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.
Since Mordred / Binary Bard only appears in the pre-recorded scenes of Astro Knights Island, you can't use the Costumizer. You may locate someone on the islands who copied the Bard when he was promoting the island in May 2009.