343 Guilty Spark is an Artificial Intelligence created by the IsoDidact. Part of him is a fragment of a human named Chakas. He is the monitor, historian, and caretaker of Installation 04, also simply known as Halo. At the time, it was known as Monitor Chakas. Monitor Chakas helped the IsoDidact fire the Halo Array. After the Forerunner-Flood War, He became known as 343 Guilty Spark. His job was to protect the Halo ring, as well as assist a Reclaimer in the event of a Flood outbreak.
Guilty Spark is first encountered at September 20, 2552, where he finds the Master Chief battling Flood in the bog outside Halo's Library. Noticing the Chief as a "Reclaimer", the Monitor transports him inside the Library.
From this point forward, Guilty Spark remains certain that the Master Chief is a Reclaimer, and so should already know all there is to know about the Flood and Forerunners. He expresses considerable surprise whenever the Chief deviates from the Forerunner's established Flood-control protocol.
343 called Chief a Reclaimer because the humans are a species to inherit Forerunner civilization. This is the reason that the prophets of Truth, Regret and Mercy start a war with humanity, as they would not want humans undermining their authority on the Forerunners.
At the end of the Human-Covenant War, it is the Master Chief that partially destroys Guilty Spark, after Spark turned on him to try and protect his newly constructed Halo, which the Chief threatened to destroy prematurely.
Between 2553 and 2557, he was interrogated by a human team to learn info on the Didact. He said the humans are "the true reclaimers".