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 purging Flood by sending them to the shelter and aegis of the Ark. The Librarian cut herself from the Flood-Forerunner conflict by building, then deactivating and further 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."

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 delayed so long the 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, whom she viewed as "special" among all of the sentient life she had seen and felt that it was well-worth the sacrifice of her life to save them.

Trivia

 * The Librarian is also an emblem you can use on Halo 3's Multiplayer, named after the character.
 * Librarian was mentioned in Halo 3 in one of the Terminals. In the conversations, she is known as "L".
 * The Librarian's name is connected to the Library because she was the one who documented all the species to the index, and that is where the index is, similar to how a modern day Librarian puts all the books in a library.
 * On one of the Terminals in Halo 3, you can see the symbol of the Librarian.
 * It is possible that she created an AI to send the creatures on the Ark back home.