Reason is a villain who was introduced in the episode "Rhyme and Reason". He is partners with Rhyme, as they have been friends for 15 years. He likes to steal valuable stuff, like gold bars or jewels, because it is reasonable. He gets upset at Rhyme because she likes to steal things simply because they rhyme with whatever he stole, as her strange crimes usually get them caught by the police.
He doesn't have any superpowers, but he can calm Rhyme down and help her attack by giving her lines to make a rhyme from.