Raiden is an unlockable character.
Metal Gear Solid: Rising lynchpin Raiden is all set to moonlight in Assassin’s Creed: Brotherhood.
Joystiq reports that you’ll unlock the Raiden skin when you’ve finished the Animus’ new virtual training mode. See the screen below for proof.
This wouldn’t be the first time there’s been some cross-fertilisation between Ubisoft and Konami’s respective franchises. Assassin’s Creed’s Altair popped up as an easter egg in Metal Gear Solid 4.