Grunt is the epitome of what a Krogan should be: violent, aggressive, and loyal.
His body and essence artificial much like a clone in ways, he is the result of the combined genetics of a handful of significant Krogan Warlords, a clone formed from more than one genetic code, if you will. For Grunt, to fight is instinct, to stop is a challenge and to fall is to die weak.
Grunt was grown in a glass tank in a lab by a Krogan scientist named Okeer. He explains that he was able to hear words and shown images while he remained somewhat aware of the surroundings and people while he was contained. Grunt is battle-ready from the moment he is released from the tank.
Adopted eventually by all the clans on Tuchanka in the trial grounds, he was originally barred from trials for acception due to his unnatural birth. The shaman who had attended the throne room of which the trials are discussed and organized accepted Grunt as a Krogan despite his current status and strength. Grunt follows and passes the trial, becoming an Urdnot clan member. At the time of initiation into clan Urdnot, Grunt was suffering urges to fight and to destroy objects, though this is common for Krogan as they mature into young adult Krogan.
Back on Tuchunka, clans have generally accepted Grunt's initiation quite well though a few figureheads and members disagree with the concept of Grunt being legitimate member of any clan.
Grunt, like most mature Krogan can bolster his natural armor and is able to shield himself with a power called Fortification.
Realistically, Grunt would be able to use most weapons given to him but a shotgun tailor made for Krogans which breaks multiple galactic safety rules for the design of a weapon, can be researched and purchased for Grunt's use. The description explaining of the violation of safety parameters would stress the recoil of firing the weapon is so great it would very easily shatter the arm of a human. However, Grunt isn't human.
Loyalty Mission: Rite of Passage
Grunt will complain to you at some point on how he is experiencing a barely controllable urge to fight, almost as though his "blood and bones" tell him to. Speaking with Grunt, you may go back to Tuchunka with Grunt and depending on which option you chose in the first game, if you carried a save over too, you will have either Wrex or his brother on the clan throne room chair.
He will speak to you and direct you to the shaman. The Shaman will have a look at Grunt's appearance, telling you he is young and that his rage is simply Grunt becoming a young adult. However, to be accepted into the clan of which he is being tested by, the Urdnot clan, he must initiate a series of tests at the Keystone monument in Tuchunka, somewhere to the outsides of the throne area. The shaman agrees that the player may fight with Grunt, an amusing renegade interruption is available during this talk, where Shepard may headbutt, much like a Krogan, the other clan leader who opposes the testing that Grunt is being sent through. This annoys him greatly but causes the shaman to burst out laughing and shouting how much he likes Shepard for doing that. During these tests, the player must activate the keystone and fight various enemies in 3 waves, ultimately battling it out against a thresher maw. It is not necessary that the thresher maw is killed, as you may wait out the 5 minutes of play but you will only earn the achievement for killing one. The voice of the Krogan speaking will explain how Krogan have survived and lived through events, which Grunt must attempt now against the thresher maw.
Grunt's voice was provided by Steven Jay Blum.