Large, suit like, bipedal robots. The Atlas are created to match humanoids to the size of Kah'Rul like a tank on legs. These machines are incredibly expensive and require expert piloting by people with incredible reflexes. They are also semi-sentient with basic Artificial Intelligence integration with capabilities to learn.
Due to the intelligence integrated into the Atlas, they can not only act autonomously, but are designed to integrate with the pilot's neural implants in order to properly synchronize movements and actions. This creates a kind of bonding side effect between the pilot and their Atlas which is universally viewed in a positive light.
The Atlas can travel through the vacuum of space for up to an hour or more depending on the loadout, allowing for their maneuverability boosters to be used for propulsion. They have optical processors in the front upper portion of their central cockpit chassis and human like limbs. Their humanoid hand design allows them to hold a variety of weapons and equipment, including using Kah'Rul sized equipment.
During a field team's encounter with a Whisper, the technology to rebuild the Atlas program was rediscovered in The Library, along with multiple sets of technological advances believed to have been lost during the war. In conjunction with the field team's engineer, the Initiative has been able to begin reviving the projects.