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. She referred to this place as "eden." When Halo was activated, she died along with rest of life in the galaxy while humanity survived on the Ark.

Her lover was Didact, commander of the Forerunner military, with whom she corresponded frequently. it is possible that it is because of this love that Didact stalled so long before firing the halos. In the end, she had to self-destruct her own ships and strand herself on earth to let Didact see what he had to do.

For some reason, she had a deep respect for humanity, who she viewed as "special" among all of the sentient life she had seen, and she felt that it was well-worth the sacrifice of her life save them.

Trivia

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