TJ-55 was a T-series tactical droid manufactured by Baktoid Combat Automata  who served in the Separatist Droid Army during the Clone Wars. In 21 BBY, he and another tactical droid were assigned to help General Whorm Loathsom of the Retail Caucus, an ally organization of the Confederacy of Independent Systems, to conquer the planet Christophsis. However, the Galactic Republic dispatched clone troopers, led by Jedi Generals Kenobi and General Skywalker, to liberate the planet. After landing, the Jedi began to prepare an ambush, but Clone Sergeant Slick, a Separatist spy embedded within the Republic forces, informed the CIS about the Republic's plan, and the location of the Republic forces was transmitted into TJ-55. Unaware of this, the Republic set up several blaster cannons on the North and South towers of the Crystal City and prepared to open fire at the droid forces as they passed through the city plaza below.
Consequently, by the time the tactical droid led his forces to the towers, he was well aware of the presence of Republic troops. He commanded his B1 battle droid forces from his AAT and surprised the Jedi and clones by splitting his troops, thus bypassing the ambush. Battle droids then attacked the South tower, where Kenobi was with his troopers. Kenobi and his clones were pinned down by the droids, but Skywalker noticed the situation from the North tower. He signaled an republic gunship for an evacuation and led his clone troopers to help Kenobi. The Republic forces were able to hold their ground but were forced to retreat with the battle droids' larger numbers. As the Jedi and remaining clones took a turbolift to the roof, where the gunship was landing for evacuation, TJ-55 left his tank and followed the Republic troopers with his droid forces.
On the roof of the South tower, the tactical droid led the attack of the battle droids, in order to prevent the retreat of the Republic troops. Equipped with an E-5 blaster rifle, he managed to shoot at least one clone trooper during the firefight. However, clone trooper Gus saw the tactical droid, jumped for him, ripped his head off, and leaped onto the Republic gunship as it departed from the tower. While the gunship left, the still-functioning body of TJ-55 rotated senselessly, shooting several mis-aimed shots on its own. Later on at the Republic's Christophsis base, TJ-55's head was analyzed in a robolobotomy by Clone Commander Cody and Clone Captain Rex. The droid's data files only verified that the Separatists had gained secret information about the positions of the Republic forces before the head stopped processing, ruining any chances of a further analysis. Despite this, the spy was discovered to be Sergeant Slick, who was apprehended, and General Loathsom's droid forces were eventually defeated as well.