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). The puzzle on the door of the castle will reveal that the Binary Bard is Mordred.
(see related question)
There is one in the middle (upper part) of the spaceship fountain next to the Mordred Museum.
You can't "costumize" (copy) Mordred in the game, but you can find users in the multi-player room that have his "cyborg" face, and copy those.
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)
You jump in between the middle and then you push them and you'll find the secret door to mordred's layer then you use the key
go to astro nights and go to the castle and find your rocket on it. Fly back to mordred
The key unlocks the entrance to Mordred's hidden underground laboratory. (You will find it under two bales of hay.)
Merlin the mechanical owl is in Mordred's lab, hidden beneath the hay bales near the Mill. (see related question)
You can find your ship on top of the castle.
Pay your admission to the Mordred Museum. In the stack of books you will find a Library Slip, which you need. Later you may also want to look under Mordred's bed for his missing journal page.
You don't see him again, as apparently he is trapped in his far galaxy. However, he is somehow transported back to Poptropica by Dr. Jupiter on Super Villain Island. By the end of that island, you have an idea how he did it.
Once you have built a new spaceship on Astro Knights Island, it will park on top of the castle whenever you leave the island.
Use the gold coin from the fountain to get the library slip from the books at the Mordred Museum. Go to the lower aisle of the castle library, and find the shelf marked McM.