Librarian

The Librarian was a Forerunner being.

She sought to document and index all sentient beings of the Milky Way galaxy and protect them from the Flood by sending them to the shelter of the Ark. The Librarian cut herself off from the Flood-Forerunner conflict by building, then deactivating and burying the portal leading to Installation 00 (The Ark). Her destruction of the remaining Keyships left her isolated on Earth. She spent her last days on Mt. Kilimanjaro, overlooking the portal to the Ark. When Halo was activated, she died along with all other sentient life in the galaxy.

Her lover was Didact, commander of the Forerunner military, with whom she corresponded frequently.

Trivia

 * The Librarian is also a symbol you can use on Halo 3's Multiplayer, named after the character.