When you defeat Mordred, the game is over, and the Princess will kayo him for you. (see the related question)
With Mordred
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Nothing. When you defeat the giant robot, you take the Orb back to Arturus and Mordred is left behind in his castle. You receive the island medallion.
You can't costumize him.
I think that it was Mordred.
It is next to the Mordred Museum at the entrance to the island.
Mordred is somewhere in space. Defeat the three planets and you can find the Golden Key (on the giant asteroid at X-11 Y-80). (see related question)
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.