Who is Merlin to King Arthur?

Usually translated as Merlin, he is a great mage. There seems to be a magic school called Merlin in Harry Potter, as if to show respect for the mage's predecessors. Merlin is a close friend of the legendary King Arthur of England. Legend has it that Merlin led King Arthur to the sword in the stone and ruled England. Merlin appeared in King Arthur and The Legend of Avalon, and should be regarded as a mentor or something.