Infiltrators were a specialized group of warriors, rigorously trained by Beri Tulon. Their primary objective was to breach and seize control of Imperial armored vehicles, with a particular focus on walkers.
When Imperial General Brenn Tantor chose to betray the Empire and join the Rebel Alliance, he pledged to General Tyr Taskeen that he would deliver an AT-AT walker to the Rebellion's forces. To accomplish this, he orchestrated the release of Colonel Beri Tulon from his imprisonment on the planet Ruul. Tulon, a former Imperial officer apprehended for attempting to defect, had previously served at the Trasse Proving Grounds, the very facility where AT-ATs were both manufactured and rigorously tested.
Tulon embraced his prior role as a training officer, enlisting and preparing a team of soldiers specifically for the purpose of infiltrating AT-ATs during the impending Mission to Trasse. These infiltrators demonstrated their capabilities, successfully capturing at least one walker for the Rebellion while also surviving the subsequent Imperial counteroffensive. They continued to serve alongside Tantor throughout his career, at least until the capture of Coruscant. Tantor frequently deployed them to commandeer enemy walkers directed against him, turning these captured vehicles against the Imperials.
The infiltrators donned combat suits that mirrored those of standard Rebel infantry, but were uniformly black in color. They were also equipped with jetpacks, enabling them to ascend rapidly to the vulnerable undersides of AT-AT walkers, where they could then cut through the armor. They were equipped with tools designed to breach the heavy armor of the walkers, and were also armed with standard-issue blaster rifles.
Each infiltrator underwent specialized training focused on the methods of entering and capturing Imperial AT-ATs. They also possessed the skills required to seize control of AT-STs, and it is likely their expertise could be adapted to infiltrate other vehicles, although their knowledge of specific vulnerabilities might have been less comprehensive. They were also trained to operate Imperial heavy armor, although they initially struggled to maneuver the large walkers with agility in combat. The infiltrators were also generally very skilled at close-quarters indoor combat and could be used to take buildings. Given these skills, it is most likely that the initial recruits were elite urban fighters trained to operate walkers, not the other way around.
The infiltrators were introduced in the game Star Wars: Force Commander to provide players with the ability to capture and utilize AT-ATs as part of the Rebel forces, as the Rebels otherwise lacked comparable heavy armor assets. They also excel as the best in-building fighters within the game, boasting superior statistics compared to stormtroopers, Rebel troopers, and Rebel heavy troopers.
It's important to distinguish these infiltrators from another distinct type of Rebel soldier also known as Infiltrators, who possessed a different history and mission profile.