From Halopedia, the Halo wiki
Keeper was part of the Librarian's crew during the expedition to Path Kethona around 98,395 BCE. He was a young, ambitious Builder who was being groomed to move high in Forerunner society; this was suspected to be the reason he was chosen to be part of the crew of the Audacity. He devoutly attended the clandestine rituals of Builder secret societies, though he was still in the earlier phases of induction into their secrets.
After the first slipspace jump toward Path Kethona he noted that they had gone farther than any other Forerunner and praised the Audacity. He reacted to the presence of ancient Forerunner and Precursor artifacts within Path Kethona with great enthusiasm, taking this as evidence of cooperation between the two millions of years earlier.
Keeper-of-Tools did not accompany the Librarian planetside after the crew discovered a world home to primitive Forerunners who had been isolated from their brethren in the Milky Way for ten million years. After the Librarian had found out the truth of the origins of the local Forerunners, Audacity and her crew returned to the ecumene, although the Librarian would not reveal what she had discovered until much later.
List of appearances