When Shepard chooses to release the super soldier from his tank, the newly awakened krogan pins Shepard to a wall and declares his intention to kill the Commander, though he desires a name before doing so. He chooses "Grunt", one of the last words in Okeer's final message; he considers it to be short and simple, descriptive of both his training and purpose. Shepard either persuades Grunt to join the mission against the Collectors as it will be a good fight for Grunt, or shoots multiple times to subdue him. If Grunt accepts Shepard's offer, he then realizes that Shepard was pointing a gun at him all along, to which Grunt approves.
Lacking a krogan upbringing or a sense of honour, Grunt is both violent and highly unpredictable. He often becomes impatient and charges recklessly into the midst of his foes, (unlike more disciplined krogan fighters like Urdnot Wrex) without Shepard's orders. Despite his unpredictable and violent attitude, he is nonetheless considered a great asset to the mission thanks to his unsurpassed physical strength and fighting prowess. Grunt also harbors personal doubt due to his genetic upbringing, considering himself weak for being given strength compared to the thousands of failures that "at least tried" to become strong on their own.
Outside of battle, Grunt often appears naive and particularly enthusiastic about violence and combat in conversations with Shepard. He often seeks guidance from Shepard due to a combination of his youth, lack of krogan guidance and his inexperience about the galaxy, having been educated solely by imprints from Okeer's tank. It takes a while for him to understand the images Okeer used to educate him with, but Grunt soon sees the funny side of killing turians and salarians, and is pleased with himself when he compiles his first list of enemies. Despite his violent nature, he respects Shepard's leadership, partly because Shepard faces great battles and has powerful enemies. If Grunt becomes loyal, he calls Shepard his battlemaster, as the Commander "has no match".
After Grunt has been freed from his tank for some time, EDI or Kelly Chambers alerts Shepard that Grunt has become anxious and is behaving irrationally. Upon talking to him, Grunt will reveal his emotions are in turmoil, describing it as similar to krogan blood frenzy and feeling confused about how to handle it. EDI suggests visiting Tuchanka to consult a krogan clan leader. The clan leader explains that Grunt is undergoing the krogan equivalent of puberty. Grunt must take part in a rite of passage with Shepard at his side to prove himself a worthy krogan and join a clan, or die in the attempt.
Mass Effect 3
Grunt makes an appearance in Mass Effect 3, provided he survived the events of Mass Effect 2. He has since earned the command of Aralakh Company. If Urdnot Wrex is alive, he notes that Aralakh Company is his best unit, and put Grunt in charge as he exemplifies the bright future of the krogan, something Wrex hopes for his people.
To a lesser extent, he still experiences the stigma of being tank-born, rather than earning strength in the way other krogan feel is true. Nonetheless, his undeniable prowess in the battle has earned him the respect of his squad.
He helps Shepard investigate the disappearance of other squads in the tunnels of Utukku, where they discover a new brood of rachni who have been twisted by Reaper technology.
If Shepard chooses to spare the Rachni Queen or artificial queen, Aralakh Company is slaughtered holding off wave after wave of rachni. Grunt personally holds several Ravagers off to cover the Commander's escape. If Grunt wasn't loyal during the assault on the Collector Base, then he will be killed; if he was, he survives and contributes to the war effort. Shepard has a chance to speak to him on Earth prior to the final battle.
If Grunt did not survive the events of the Collector attacks, his role is filled by Urdnot Dagg.