Zealots

"Oh, I like them. They were funny."

- Caboose

The Grunts were two teams of Red and Blue soldiers living in an unknown part of the galaxy. These soldiers were for some reason in a constant loop of attack and retaliation. The two teams would senselessly kill each other until only one was left. And then that one would kill himself. Somehow the song Reveille would bring these soldiers back to life and they would rush back to their respective bases to await the next battle. The blue grunts also have their motto as "Kill the reds, kill the reds!" and run out after Reveille is played. The only identified Grunt was the Red Zealot. It has been confirmed that the Grunts are a part of the Red and Blue simulation set up by Project Freelancer.

Battle at Battle Creek
The Grunts' religious figure seems to be a flag held in their bases at Battle Creek. When The Red Zealot captured the blue flag the blue team focused their efforts on killing the soldier and restoring the flag to its rightful place. The Red Zealot also seemed to believe in "The Anti-Flag" (which was actually Caboose), and believed it would rule them for seven years. When Sarge and Caboose arrived at the battlefield they were caught in the crossfire of the opposing teams. The two exploited the local religion in order to escape, or as Sarge put it "getting ourselves completely involved and kicking some serious ass!"

Arrival at Sidewinder
The Grunts were later summoned to Sidewinder by Simmons by using a teleporting device. Simmons manages to trick them into fighting O'Malley by saying that he has the flag. The battle ends when the bomb inside Church detonates which sends nearly all the Grunts, except the Red Zealot, to a distant location within a snow valley.

Alliance with Wyoming
While at the base within a frozen valley, the Grunts apparently came under the service of Wyoming and acted as his bodyguards. When Tex, Tucker, Caboose, Andy and the Alien come to the base in search of the treasure, Tex attempts to use stealth in order to kill the Grunts and get to the treasure. However they are soon alerted to their presence when Tucker makes a small noise. Tex soon kills them all single handedly with a pair of SMGs.

Afterwards, as Tex continued her pursuit for Wyoming, she, along with York and Delta, eliminated the remaining Grunts in a firefight, atop Omega's fortress. Two dead Grunts are later seen in Recovery One: Part One when Washington arrives to collect Delta.

Trivia

 * The Grunts were created by Burnie Burns as a parody of stereotypical Halo gamers and multiplayer variants.
 * It's currently unknown how and why they revive upon hearing Reveille.