After completing the mission Dossier: The Warlord, you will have to decide what to do with Dr Okeer’s tank-bred Krogan Grunt.
Choice: Open the Tank or Keep it Sealed
During the Dossier: The Warlord mission, you will meet Dr Okeer who has been trying to breed the perfect Krogan. After a firefight and before he dies, he asks you to save his research which you recover to the Normandy. After Miranda and Jacob voice their concerns about what to do with the Krogan, you can visit the Krogan in the cargo hold where you can opt to either open the tank or keep it sealed.
Open the Tank
If you decide to ignore the warnings of EDI and open the tank, you will be confronted by the Krogan who names himself Grunt. Appeasing him during the confrontation will result in him joining the Normandy’s crew and will unlock him as a full squad mate, complete with Loyalty Mission and storylines for Mass Effect 2 and 3. If you wish to gain The Krogan achievement for recruiting Grunt, it goes without saying you should choose this option. He also makes a badass companion so there is also that.
Keep the Tank Sealed
If you decide to keep the tank sealed throughout Mass Effect 2, he will not be unlocked as a companion, nor will he appear in Mass Effect 3 if you import that save. For the duration of Mass Effect 3, he will be effectively dead and his Loyalty Mission and storyline will be locked to you.