Rank
Minors are the lowest rank in the Grunt ranks, essentially the cannon fodder of cannon fodder. Bearing light-orange methane rebreathers, Elites often send them into battle with little thought. They are usually Infantry, at most Ghost pilots.
Combat
Usually found brandishing Plasma Pistols, Needlers, Plasma Rifles (if you give one to a grunt as the Arbiter), or rarely Fuel Rod Cannons, these Grunts are the most cowardly beings in all of the Covenant ranks and are an easy kill. Taking out an Elite overseer or a Major Grunt will usually cause all Minor Grunts in the area to lose discipline and flee. Minor Grunts are easy to kill and are low priority. However, a player shouldn't underestimate even a Minor on Heroic or Legendary. They are quite good shots with their preferred Plasma Pistol, and if in a squad, will concentrate their fire on the player, bringing down their shields quickly. Many techniques are used to kill Grunts. Generally the most effective method is to use either a Battle Rifle or SMG. Use headshots over medium distances with a Battle Rifle, or get in close with an SMG and take them out with rapid gunfire. The Grunt's armor gives little protection to UNSC ballistic fire, and if hit by a bullet a Grunt Minor may flinch and stumble back. It is then easy to go in quick with a melee attack, or pin them down with continued fire. Minor Grunts are particularly vulnerable to explosives. A single grenade a few feet away from the Grunt will kill it, though if they are too far, but still get hit, they will stumble around with their arms over their head for a short period of time, before running away at top speed. It has also been noted that when a large group of Minor Grunts form, a Rocket Launcher is one of the most effective weapons, and would most likely kill all of them, do to their low resistance to weaponry in general. Also, Sniper Rifle rounds can kill a few Grunts at once, if used correctly and at the right angle. Basically, the only weapon that might have a hard time killing Minor Grunts would have to be the Plasma Pistol from Halo 2, but even then, it can kill them quickly if you knew how to use one.
Minor Grunts are usually led by a Minor Domo or a Grunt Major. They will usually run in fear and attempt to find cover if their leader(s) have died, though they will often regain composure and attempt to shoot after ten to fifteen seconds.
Killing Elites causes Minor Grunts to flee, but if there are higher ranking Grunts nearby, they'll calm down their little brothers, and the Minors won't be happy...
Appearances
Minors are perhaps (with maybe the exception of Flood Infection Forms) the most common enemy in the Halo series.
Halo: Combat Evolved
Minor Grunts appear in almost every level in Halo: CE, except for The Maw and The Library.
Halo 2
Minor Grunts appear in Halo 2 at about the same frequency they appear in Halo: CE. They appear in most levels, except for a few exceptions, like The Arbiter (Level) and The Oracle.
Grunt minors usually attack in groups of three to ten, like other ranks of Grunts, and do not appear that often on Legendary, as most of them are replaced by Grunt Majors to increase the difficulty of the game.