Déjà was a "dumb" AI who trained the SPARTAN-IIs in their first stages of training at the CASTLE Base and Reach Naval Academy facilities on Reach. Her educational lessons were part of the Spartan's training program, meant to increase their strategic thinking. She also educated them in history, mathematics, and other common subjects.
In one of Déjà's first lessons, she taught the Spartan-IIs about the famous Battle of Thermopylae, where 300 Spartans made a stand against a quarter of a million Persians. Another lesson was about seven wolves taking down a moose, by working as a team. During one of Mendez's training missions, she helped the Spartans out when John-117 called her for assistance in returning. Using a mail link, he managed to link her to a Pelican dropship to request she show him how to fly it to get the Spartans home. While she couldn't do that, she had him activate the autopilot, allowing her to take control of the craft and fly the Spartans home herself remotely.
Her counterpart, Chief Petty Officer Franklin Mendez, gave the Spartans rigorous physical training to make them into soldiers. This, along with Dr. Catherine Halsey's physical augmentations and MJOLNIR armor, made the Spartans intelligent, strong, and fast.
Déjà also acted as a sounding board for Dr. Halsey, discussing how the Spartan-II Program should be run. For example, the night the program officially began she advised Halsey on how to address the children, at one point persuading the doctor not to delay the program in case ONI decided to replace her with someone less qualified to the childrens' detriment, a moment that suggests that Halsey may not be as selfish as many, herself included, believe. Later they would discuss whether Halsey was giving Soren-066 the choice to proceed in the program in order to alleviate her own conscience.
- Halo: The Fall of Reach (First appearance)
- Halo: Ghosts of Onyx
- Halo: Evolutions - Essential Tales of the Halo Universe