A major theme of the Mass Effect universe is creating your own future rather than being bound to fate. This is a lesson that the Geth learned well. Grunt, the perfect Krogan, learned it too.
Grunt was born to be the best of the best, but he was also designed to be warlord Okeer's puppet and template for a reborn Krogan race that he could control. But this didn't pan out. Okeer died before Grunt was "born," and with no Krogan master by his side, Grunt had to figure out everything for himself. However, at least he had Commander Shepard to lead him.
Lonely At The Top
Commander Shepard happened across Grunt during a mission to recruit warlord Okeer for the Normandy's elite team in Mass Effect 2. Shepard visited the savage scrapyard world of Korlus, a place overrun by rogue Krogan who kept emerging from a fortified laboratory. Warlord Okeer explained to Shepard that no cure for the genophage was necessary. Instead, the Krogan would condense themselves into an smaller but tougher army with no weak links, and the tank-born Grunt would serve as a template. Okeeer died when a gang leader named Jedore tried to sabotage his project, and Shepard departed with Grunt in his tank. Then, Grunt is awoken aboard the Normandy, in the cargo hold.
Grunt had all kinds of memories and ideas imprinted into his mind via Okeer, but he took no interest in Okeer's personal mission. Grunt was starting over, and he defaulted to the first warrior-leader he found: Commander Shepard. At first, he felt directionless as he fought by Shepard's side, since he longed to fight but had no clear cause. He was an elite fighter, born with the assembled genes of his race's finest, but he was otherwise empty and was perfectly willing to admit it. Then, he felt the itch of early adulthood, meaning he needed to undergo the rite of passage on the Krogan homeworld of Tuchanka. To keep Grunt happy and focused, Shepard visited that harsh world, and there, Grunt's journey of self-discovery truly began.
Finding His Way
If Urdnot Wrex survived the Virmire mission, then Shepard will find him on Tuchanka as the leader of the ever-growing Clan Urdnot. He explains that Grunt isn't suffering a strange medical condition; he just needs a proper release for his pent-up rage and bloodthirst.
After speaking with the clan's shaman, Grunt is allowed to begin the trial, but already, he's ruffling some feathers. Gatatog Uvenk is a traditionally minded Krogan who scorns Grunt's very nature, but once Grunt completes his combat trial, the scheming Uvenk offers him a place in his personal army. Grunt refuses, being insulted by Uvenk's selfish scheming, and he only acknowledges Commander Shepard as a true battlemaster. Grunt and Shepard wipe out Uvenk and his personal guard, and Grunt officially becomes an Urdnot.
Bit by bit, Grunt assimilates into the ways of the Krogan, and he enthusiastically follows his human battlemaster into action against the deadly Collectors. If Grunt survives the final mission to the Collector base, he will part ways with Shepard and learn more about the ways of the Krogan on Tuchanka. Months later, he has become the young leader of the elite Aralakh Company, a unit of the Krogan race's finest warriors all from different clans, which represents newfound Krogan unity.
Shepard will cross paths with Grunt on a desolate world where husk Rachni are holed up, and they journey into the world's caves to find the Rachni queen. These creatures are the Krogan race's natural enemies, but Shepard may choose to spare the Rachni queen and enlist her aid in the Crucible project. This comes at a cost; Grunt will lose all his men fighting off the Rachni husks to evacuate the queen and protect Shepard's squad, but he will comply with Shepard's wishes. By the final battle in London, Grunt has fully realized his place in Krogan society. He is a warrior with clear purpose and honor, and he didn't need puppetmasters like Okeer or Gatatog Uvenk to do it. He needed friends and faith in himself instead.