He is the arch-nemesis of Guilty Guilty. Let me tell you the tale, the true tale, that can be heared nowhere else.
Originally, there were twelve Halo rings. This means there were twelve Monitors, and Greenie was one of those Monitors. Like most of the other Monitors he had developed a quirk, with 2401 Penitent Tangent it was lunacy, with 049 Abject Testament it was amnesia, and with 343 Guilty Spark it was dedication. But 007 Greenie Spark developed something altogether different. He developed a vision. A vision of a Galaxy free of Forerunner dictation, free of the Didact and free of the Prometheans. And as the Halo rings were bein constructed, he formulated plans to achieve this vision.
When the Monitors met for the first time, friendships and rivalries were commonplace. Guilty and Greenie developed an instant disliking of each other, and they often quabbled over what colour the ring's interior design should be. This was just the beginning. Greenie and Guilty fought over everything. The sizes of the rings, the colour of the monitors, and even the kind of socks the Forerunners wore. (No, seriously, Greenie wanted spots, Guilty wanted stripes).
Eventually, all this squabbling culminated in all-out war. Greenie went against the other Foreruner Monitors over the firing of the Halo Array, and rebelled against them. Around this time the Janitor had just developed a new 'warrior' faction for the Forerunners, and Greenie and Guilty used these new 'Sentinels' to battle it our across twelve Halo rings. This conflict lasted for twenty-two years, and the eventual outcome was a splitting of the Monitor factions.
The majority of the Monitors sided with Guilty, who seemed more sane. However, Greenie's insanity stemmed from desperation to save all life in the Galaxy. He and four other Monitors returned to their Halo Rings and set them on a random vector course, using algorithms the Forerunners could not comprehend in time, and the five rings were lost, leaving only seven to be fired. This meant that the Forerunners could not carry out their ultimate plan to cleanse the Dark Spaces in which Gravemind and Harbinger resided. The other four Monitors were never seen or heard from again, and although the ruins of Halo Installation 09 were found in the outskirts of the Force Galaxy centuries later, the Rebel Monitors seemed to have abandoned the Galaxy forever.
However, this was not the last that the Galaxy heard of Greenie. Although he and his physical form went with his Halo (the original Installation 01), he duplicated his personality and implanted his thoughts into the Forerunner Domain, and he slowly began to influence the Librarian, causing her to chage her mind and try to save all life in the Galaxy, creating the Ark to document specimens of every species that would be affected by the rings. This made the Didact pretty mad, and caused him to go insane. Greenie's personality remained in the Forerunner domain, and caused Guilty Guilty's surprising signal loss during his 100,000 year stay on Installation 04 (previously Installation 12). Greenie remained in the domain until the death of the Didact, during which he rescued and preserved a fragment of the AI Cortana, slowly nurturing her back to health. He learned from her that Guilty Guilty had met his maker at the hands of Johnny, and was grateful for this.
Cortana and Greenie remained together for many years, shared data and syncronisig their histories. As Cortana slowly recovered, Greenie found that his own mind was healing, and began to make plans for their triumphant return home. Greenie Spark wrestled control over the Prometheans from the Foreunners and forged himself a new body, calling himself the Warden Eternal.