From Halopedia, the Halo wiki
Lifeworkers were a rate in Forerunner society. They specialized in all living things and medicine, including experimenting with new forms of life. Paradoxically, despite the ideological basis of the Forerunners' Mantle to protect all life, the Lifeworkers were rated rather low, below Miners but above Warrior-Servants. The highest possible rank achievable by a Lifeworker was Lifeshaper. The most prominent Lifeworker was the Librarian, a Lifeshaper and one of the most powerful Lifeworkers to have ever lived.
The Lifeworkers were responsible for creating and maintaining the ecosystems on Installation 00 and the Halo Array, this being part of the Librarian's pact with Faber, the Master Builder, providing a contingency plan for sentient species in the event the Halo rings were used. The ecosystems were used by the Lifeworkers to research, store and index species collected from numerous worlds across the galaxy in the Conservation Measure. For nine thousand years, after the end of the human-Forerunner wars, Earth, then called "Erde-Tyrene", was an experimental planet under the care of the Librarian, who ensured that humanity would recover from their forced regression and prepared them for their role as Reclaimers with her geas.
Lifeworker forms were typically graceful and tall compared to other rates. Some types of Lifeworker armor enabled floating as opposed to walking. This is seen quite prominently with the Librarian and her Lifeworkers on Omega Halo. Senior Lifeworkers were able to attain the title of Rchmedicus.
List of appearances