Librarian

"And what about us, Didact? We've been irresistible and immovable for too long. Maybe it's our turn to give."

- The Librarian to Didact

The Librarian was a Forerunner being who's true name is unknown.

She sought to document and index all sentient beings of the Milky Way galaxy, and protect them from being absorbed into the Flood by sending them to the shelter and aegis 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". This is probably a reference to the Garden of Eden, or may in fact be hinting that the Librarian was in fact Eve of Bible lore, and the mother of humanity.

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 of 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 held a deep respect for humanity, whom she viewed as "special" amongst all of the sentient life she had seen and felt that it was well-worth the sacrifice of her life to save them.

Quotes
(As portrayed through the Terminals)
 * Categorization has sped since the improvements were announced, but there are many hurdles. The indexing of sentient species may have irreversible effects on the surviving insentient species. We will have extinction events and irreparable environmental harm on at least 18 worlds. Current projections estimate post-archival cataclysm on as many as 31 worlds. The paucity of sentience has been a blessing in this regard.


 * Would that it were my choice. I have committed to this course because it's the right thing to do. We no longer have the manpower or material to excise remedial measures at a planetary level. I certainly can't justify using the [transit measure] to save my own skin when there are still so many innocents to protect and index.

And I'm so close. Close to saving them all.
 * We have no time to spare, Didact. Every vessel we can fill, we send to the Ark. I dare not cease the mission. Not now, not until I've done all I can. Each one of these souls is finite and precious.

Please. Activate the Array.
 * I'm close to finishing the task. The indexing and the archival processes are as complete as I can hope for. If we wait longer, we risk catastrophe. The thing has already destroyed every colony on my side of the line.

Can't you see? Belief in the Mantle sealed our doom! Weakened our [protectorates], bred dependence and sloth. Our [so-called Guardianship] has stripped those we would keep safe of any capacity for self-defense! Were we such noble [Guardians] when we drew our line and abandoned billions to the parasite?
 * The Mantle? You still hold to that [belief] after all that has happened? After this thing has consumed a million worlds?


 * And what about us, Didact? We've been irresistible and immovable for too long. Maybe it's our turn to give.

Do not let your concern for my welfare commit you to this suicidal scheme! It's coming for you.
 * Are you insane? Would you risk every life in the galaxy for this transparently futile plan? Have you learned nothing in these last [300 years[?]]? The thing will laugh at your efforts!
 * Something is wrong! It's moving away! At night I can see it--flitting shadows--black against the stars. Thousands of ships! Not spiraling outward, but heading for the line! This is the tipping point, Didact. It's no longer feeding.

I'm trapped. On a beautiful, empty world. Its inhabitants have been safely indexed, every single one of them. They're special--well worth the effort it took to build one final gateway even at this late hour. This may be our last communication. I'm begging you. Fire the Array. Light the weapon, and let it be done.
 * I've remotely destroyed our Keyships. A security measure. Without them I cannot reach the Ark. But neither then can the thing.

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.