Flint first appears while Aya Brea is staying in Dryfield. Flint will help Aya in more ways than one throughout the game. Flint is obedient and smart as seen as he leads Aya to the underground tunnels so that she could investigate a scream that she and Gary heard earlier.
During the night the Burner attacks, if Aya doesn't deal with it quickly, it will stomp off killing Flint in the process. If Aya does take care of the creature, Flint will survive and stay with Gary in the desert.
If Flint survives, Flint later appears near the end of the game at the military compound with a canister attached to his collar along with a note from Gary. Aya must give them the teddy bear scent and Flint will locate a wounded and blinded marine. Saving this marine is required for the good ending -- when Aya leaves Flint, Flint is sent back to the surface in order to lead this blinded Marine to safety.