Reclaimer

A "Reclaimer" is an individual who, whether by chance or by precedent, has been placed in charge of retrieving an item known as the Index, returning it to, and subsequently placing it into a control panel within a control center on a Halo structure to activate a massive superweapon. The word is first used as a title bestowed upon a Staff Sergeant Marvin Mobuto in Halo: The Flood and Master Chief in Halo: Combat Evolved by an artificial intelligence construct, designated 343 Guilty Spark by its creators, the Forerunners.

History
The only known role of a Reclaimer is to recover the "Index" and return it to the Halo control center. Doing this would activate Halo's ability to destroy all sentient life within twenty-five thousand light years, and would also activate other Halos, causing the entire galaxy to be purged of sentient life. This would be done to prevent the spread of The Flood, which can only be accomplished by destroying all sentient life with a biomass sufficient enough to sustain them. This would then destroy the Flood via starvation.

343 Guilty Spark assumed, for unknown reasons, that the human soldier Master Chief was a Reclaimer. However, Commander Keyes and Sergeant Johnson were also called Reclaimers by 343 Guilty Spark. Penitent Tangent also refers to Master Chief as a Reclaimer, implying that all Monitors recognize him as such. In addition, in Halo: The Flood the Master Chief encounters the corpse of Sergeant Marvin Mobuto in the Library, whom Spark also refers to as a Reclaimer.

Other Interpretations and Speculation
''The above is one possible interpretation. It is not stated explicitly in either gameplay or literature that Reclaimer is even a title. Neither The Master Chief nor Commander Keyes are ever referred to as the Reclaimer. The Monitor always uses the terms Reclaimer, a Reclaimer, or this Reclaimer.''

Reclaimers are literally Reclaimers
Although only briefly referred to in Ghosts Of Onyx, one of the Sentinels speaks and refers to one of the Spartan-III's as a Reclaimer. This might mean that all Forerunners involved in the Halo project were Reclaimers, or the people that hid in the shield were meant to come back and "reclaim" all of the Forerunners technology after the Halo system fired.

Also, after the Spartan-III responds, the Sentinel reclassifies him as an "aboriginal sub-species." One could argue that the Spartan-III's are a lesser form, or sub-species, of the Spartan-IIs. However the more likely thing that happened was that the Spartan-III simply did not give the Sentinel the correct response.

It was stated in Episode 5 of Iris that humans are meant to Reclaim the Ark and any other Forerunner technology.

Reclaimer is another word for human
Another interpretation is that a Reclaimer is the Forerunner terminology for a human. From this it is possible to assume that the Forerunners had knowledge of humans and delegated to them the task of reclaiming Indices and activating the rings, among many other speculative duties. This theory is supported by the scene when an Onyx Sentinel chasing Spartan-III G099 attempted to communicate with him, calling him a Reclaimer. Also, the Brute Tartarus ordered Miranda Keyes to insert the Index, instead of doing it himself, further supporting this theory.

The recent conclusion of the Iris ARG hinted that humans are reclaimers because the Forerunners discovered humanity soon before they activated the Halo array, choosing to protect the humans rather than let them die out too.

Forerunners are human
An interpratation of Reclaimer has been that humans were in fact Forerunners due to the comments said by 343 Guilty Spark and also due to similarities with the Halo installations and Earth and with the Ark being on Earth. But with the recent release of Iris it was confirmed when Episode 5 opened that humans were not Forerunners. This was also comfirmed in the online comic Cradle of Life showing N'Chala watching the Forerunners build the Ark.

Enhanced means Reclaimer
Another interpretation is that is not the species, but their cybernetic implants that mark the Master Chief and Commander Keyes as Reclaimers. Gravemind also referred to the Master Chief's "machine and nerve" mind as "concluded", which may mean it is closer to some arbitrary standard (the Forerunners?) than a merely organic mind. As all UNSC personnel have cybernetic implants of varying qualities, this may mean that a civilian human would not be regarded as a Reclaimer. This is, as is much of the Halo universe, all highly speculative.

(Given the situation, machine and nerve could simply be a reference to his cybernetic enhancements and has his mind 'concluded' in ending this war, while the Arbiter was 'merely flesh and faith and is the more deluded' in that he had no modifications and was blind in his faith towards the Covenant beliefs)

Forerunners evolved into humans
The small comic strip "The Cradle of Life", disproved this theory, since it shows a Human tribe elder watch as the Forerunners build the Ark.


 * But the tribal elder refers to them as the Gods. So they might have been going there for thousands of years. So then they may still be the evolved as the Forerunners may have been controlling the early forms of humans.

However it could be that the entire Halo story is actually story that keeps repeating itself and so the halos are destroyed by the chief then years later humans build new ones, encounter the flood and the whole thing reapeats. It could be that the master chief ends up breaking the halo chain and ends up reconfiguring the Halos to only target The Flood. However this is only speculation.

Another theory is that the Forerunners encountered primitive humans while building the Ark and decided to make sure that humans gained intelligence after the Halos are fired and the Forerunners are killed along with all other intelligent life of sufficient biomass.

It was confirmed in Episode 5 of Iris that the Ark was built to protect humanity and that Forerunners had no previous interaction with humans.

Time Travel
Working with the same quote, from the Two Betrayals (Levels). "Why do you hesitate to do what you've already done?" this line perhaps suggests that as a Reclaimer, the Chief would be referred to as a Forerunner so therefore has done it generally as a race.

One other speculation on the Halo 3 plot would be that the Master Chief will time travel 100 000 years ago (Maybe with the help of some Forerunner technology) to activate the ring and try to eliminate the Flood. A significant possibility is that the Gravemind will travel back in time to achieve some sort of goal, and the Chief (and possibly the Arbiter) will follow it in order to prevent it from achieving its goal. He will do this on Installation 04 with help of his monitor, 343 Guilty Spark. Then he may stay in this time (and obviously die, as he won't be able to go to the Shield World, assuming he is not remotely activating the ring from the Ark), or maybe go back in 2552 just after reunified the index with the core, and just before the final activation of the ring). It could explain why the monitor tell MC "Why do you hesitate to do what you've already done?" : Because the MC actually did it already. This is clearly a speculation, but the fact that Halo 3's plot will implicate time traveling is very likely. This would also explain why the Forerunners did not relocate to the Shield World, as they did not have any clue of the Halos' firing.

The time travel theory is supported by similarity to Bungie's earlier trilogy, Marathon. In Marathon, the main character, an advanced super-soldier, fights an alien race dead set on humanity's destruction, but beset by civil war. This super-soldier has assistance from AIs, and there are hints of a long-vanished advanced race with a legacy of destruction. This story is very similar to that of Halo, and if one were to apply Marathon's finale to the Halo trilogy, then the Master Chief would travel back in time, just as the main character in Marathon does, in order to stop the enemy alliance from releasing a chaotic force that threatens to consume all life in the galaxy (The Flood). If the Halo story follows Marathon's to the letter, it may mean that the Master Chief does not travel 100,000 years into the past, but rather only a few weeks into the past, so as to stop the Covenant from releasing the Flood on Installation 04. Evidence to support this theory comes from the E3 2007 trailer, which briefly shows a number of human and Covenant ships in battle above another Halo. This could be the battle above Installation 04 from Halo 1, as it is unlikely, assuming the Chief travels through time, that an entire fleet of human and Covenant ships would follow him. Also, this would create a paradox, so it is entirely likely that if this does happen, it will fail. However, it could be that this new Halo from the trailer is simply one of the six other Halos in the galaxy; this is all theory and speculation. None of this has been confirmed or denied by Bungie, so the overall plot for Halo 3 remains a mystery. but slightly possible because the forerunner's had the ability to go forward in time via shield world.

Insanity
However, the above can be contradicted with the fact that 343 Guilty Spark is arguably insane. When he made the quote "Why do you hesitate to do what you've already done?" and recited a previous conversation he had with an unnamed individual, Spark may very well have been referring to a different Reclaimer from when Halo was first activated and, due to his insanity, conceived the Master Chief as the original Reclaimer. Or it could be that the Chief already put the index in and tried to activate the ring. However Cortana took the index from inside the mainframe and did not allow Halo to be activated. So when 343 Guilty Spark says "Why do you hesitate to do what you've already done?" it is referring to the Chief already trying to activate Halo. This too, however can be countered by the fact that Guilty Spark then goes to say "Last time when you asked me, 'would I do it?' Having had considerable time to ponder your query, my answer has not changed. We must activate the rings." he does in fact believe he has encountered the Master Chief before, or someone who greatly resembles him.

A problem with this theory is that the monitor of Installation 05 also referred to MC, and not the Arbiter, as a Reclaimer. This occurs during the cut-scene where Gravemind is introduced. Instead, the Covenant, or possibly just the Elites, are referred to as "Meddlers," taking a quote from the final scene in The Oracle, where Spark says "Why do you meddlers insist on using such inaccurate verbiage?," to the Arbiter