Rojo is the leader of a heavily-armed female biker gang.
When Rojo tried to rob an armored truck, she is thwarted by Four Arms and two of Vilgax's Drones coming after him. The remnants of one of Vilgax's defeated drones merges with Rojo as she investigates it, and she becomes a powerful cyborg. The upgrade gives her the ability to fly, superhuman speed, metallic claws, and a pair of blasters attached to each shoulder. Vilgax is able to communicate with her through the parts and force her to track down the Omnitrix for him. Rojo is turned back to normal when Upgrade merged with her, took over her body and shorted-out the spare parts, after which Gwen knocks her unconscious for attempting a sneak attack on Upgrade.
Rojo is greedy, wild, amoral, and not hesitant to attack innocent bystanders just because they annoy her. Neither her real name or her cyborg name is mentioned in the show, except for in Ben 10 vs.the Negative 10 but are listed as such in the credits.
Rojo first appears in the episode The Alliance. She returns in Ben 10 vs. the Negative 10, as part of the Negative Ten, with new alien tech and armor; presumably supplied by the Forever Knights. In that episode she is also shown to have a rivalry with Charmcaster over being the "baddest girl" in the group. In the battle, she interferes with Gwen and Charmcaster's duel but gets kicked away by Charmcaster and Gwen's combined spell power. Later she quarrels with other Negative 10 members and later was trapped in the force field where the sub-energy once resided.
Rojo returned in the Ben 10: Ultimate Alien episode Hit 'Em Where They Live, where she and her gang tried to rob a train. She apparently reformed her biker gang and upgraded them, using blue and yellow versions of her new red high-lighted suit. They were eventually stopped by Rath and the gang. Rojo now has darker skin and shoulder-length hair, brighter red than Gwen's.
Rojo is Spanish for red, which explains her red armor. But her villain name is not pronounced in the series using Spanish phonetic rules. In Spanish, the letter J makes the H sound, making Rojo pronounced as "ROE-hoe" in Spanish, not "ROE-joe".
Rojo is short form for Robot Joey.
Rojo is one of the few villains who has two different appearances in the original series.
According to Derrick, there are plans for Rojo in Omniverse.