It's more than likely a Dark Side of Dimensions character. There isn't really another reason they would give no hints as to their identity, unless it's an early Zexal character or something equally unlikely.
Based on the Key Keeper comment my guess is Atticus/Nightshroud. He frequently referred to Jaden as key keeper in their first duel as he had a spirit key. The only other characters I can think of involving keys is Shadi with the millennium key and Yuma with the Emperor's key.