Dark Roy Koopa looks identical to the real Roy, but has a dark shade to his sprite.
Dark Roy Koopa shares the same abilities as Roy, (such as his Spin to Win ability).
Minion Quest: The Search for Bowser
Dark Roy Koopa first appears in Minion Quest: The Search for Bowser. After Roy has been recruited, Dark Roy Koopa replaces him in the missions the player had to fight the real Roy.
Bowser Jr.'s Journey
Dark Roy Koopa returns once again in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. He appears in the final battle against the Best Fitness Friends (along with the other Dark Koopalings). In the final battle in the post-game, he is fought again, but this time, under the effects of Malatone Formula:X.