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 and then deactivating a portal leading to Installation 00 (The Ark), leaving her isolated on Earth. When Halo was activated, all life on Earth were somehow spared.

Her lover was Didact, commander of the Forerunner military.

Trivia

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