From Sonic Retro
Sir Galahad is an alternate reality version of Silver the Hedgehog available in the multiplayer mode of Sonic and the Black Knight. Unlike Sir Percival, Sir Lancelot, or Sir Gawain, Galahad plays no part in the game's story mode. He and Sir Lamorak are presumed (but never explicitly stated) to have been part of the Black Knight's Knights of the Round Table.
Sir Galahad is unlocked for multiplayer by beating the Lancelot Returns boss in Deep Woods. In Battle Mode, he does not actually physically hold his sword, instead wielding it via psychokinesis in a manner mimicing the abilities of his dimensional couterpart.