Rouge the Bat is an undercover spy working for G.U.N (Guardian Unit of Nations) to uncover the secrets behind Project Shadow and the Incident 50 years ago on Space Colony Ark. To do this Rouge temporarily joins Dr. Eggman and Shadow the Hedgehog in their quest for world domination. Rouge while spying in disguise, is also an infamous jewel theif. Rouge has a thing for the Master Emerald as she is constantly trying to take it from Knuckles the Echidna, Guardian of Angel Island, and the Emerald as well.
Rouge and Knuckles have had many incounters and fights with each other over the Master Emerald. This rivalry has continued in many of the games, and is self evident through Rouge's love for all sorts of treasure.