Grunts, otherwise known as Unggoy, are the lowest of the low in the Covenant hierarchy. They have combat training and are commonly used as cannon fodder in battles, but even then they are basically worthless. Their numbers make up for their strength. However, they would rather run from battle and save their own lives, although their courage is usually brought about by that of their Elite (and Brute) leaders. They generally wield plasma pistols, but the higher ranking ones can be seen with fuel rod cannons. They are also known to be coward and can be real talkative in contrast to other Covenant species. They were first introduced in Halo: Combat Evolved in the very first level.
The Grunts were introduced to the Covenant empire long ago, hundred of years before the Human-Covenant War. They were enslaved and treated in a bad and low manner, many Grunts were used in suicidal duties and dangerous missions. The Grunts tried to rebel against the Covenant Empire lots of times to gain respect and recognition, only to have their homeworld Balaho in danger and under threat of destruction and being glassed.
As mentioned, the Grunts are extremely weak as individuals and when unarmed, even with strong weapons such as Rod Cannons, they still lack endurance and strength. Although if a massive amount of fellow Grunts and powerful leaders are present, motivations can raise alongside with morale, resulting in stronger Grunts. The Grunts move on both feets like Humans with a slight spine bend, their speed is moderate and well balanced. Grunts have a large pair of arms similar to those seen in Gorillas, it functions as limb to walk on in fleeing and danger state. The Grunts tend to socialize, interact, and give out opinions more than any other known outer-space species. They also interact with each other in various activities, even in war fields.
The Grunts are not capable of breathing oxygen, and might get choked if inhaled it for a long period. They breath methane instead, and use special masks and devices that transform and convert the oxygen to a breathable methane when on earth or other Human facilities. Basing on this, it can be assumed that their homeworld lacks oxygen gas, and methane might be diversely present. And this might explain the reason why they have an odd blue blood.
Although it is unknown how to identify the genders of Grunts, they are assumed to be males despite their weak behavior at war. If compared to humans, they seem to have a mental behavior similar to Human children. They reproduce in means of sexual methods and interbreed occasionally with successful fertile offspring, though it is not proved but asexual reproduction in Grunts seems illogical.
The Grunts are some of the funniest creatures in any game. With the I would have been your daddy Skull, and the grunt birthday party skull, The Grunts are a walking Laugh bomb. They make funny comments and they have some Individuals, like the Jerk store Grunt. In some occasions in Halo 2, when you make contact with one and retreat or don't make an offensive move, they will refer to you as being "Coward" and order to fight back. Probably another reason for their popularity besides their humorous behavior, is their size and the fact that they are easy to kill. They are also recognized for the large pointing hood that sticks out of their back, they are also seen in a state of rest "Sleeping" throughout the Halo games.
The Covenant Grunts may not be the most proficient of fighters, but on rare occasions, they are tasked with more complex tasks. On harder difficulties you will sometimes find grunts wielding Fuel Rod Guns and in some cases even driving Warthogs or Ghosts. They also serve as the basic attack infantry units. In Halo Wars there is a suicide Grunt unit in which his role is to detonate an explosion causing damage and self death, notably because their value to the Covenant Empire is extremely low.
The Grunts are extremely weak, but they shouldn't be underestimated, in some cases they can as deadly as Elites can be. Generally, the most easiest way to take one out is performing a melee attack on him. Grenades aren't useful and using them is just a waste, unless the Plasma Grenade gets stick to their body.