Gary Mitchell was a Starfleet officer during the 23rd century of the alternate reality created by Nero's 2233 incursion. Mitchell scored highest of all USS Enterprise crewmembers in terms of esper, apperception, and Duke-Heidelberg Quotient tests.
Mitchell attended Starfleet Academy as a member of the class of 2257. At the academy, he assisted James T. Kirk with his homework assignments. When Kirk was made captain of the in 2258, Kirk's first request was to bring Lee Kelso and Gary Mitchell aboard.
Soon after, Mitchell beat Kirk in a game of three-dimensional chess before Kirk called Lee Kelso and Mitchell to the bridge to relieve an exhausted Hikaru Sulu and Pavel Chekov during the SS Valiant investigation. When the Enterprise attempted to cross the galactic barrier, Gary Mitchell's console exploded, but he remained uninjured. At that time, the effects of the barrier took hold of Mitchell.
Gary Mitchell developed a fast reading ability and ability to absorb knowledge, along with telekinesis and increased strength. While sedated, Spock performed a mind meld with Mitchell and determined that Mitchell has "no consciousness" and "no sentience of any kind". The apparent mutation showed as his eyes turned totally black, then opaque, then with with a cloudy blue color. Kirk ordered the senior staff to not discuss the situation with the other Enterprise crewmembers and for Leonard McCoy to keep him informed about Mitchell's status.
Eventually Spock recommended leaving Mitchell on Delta Vega I or killing Mitchell before the Enterprise met the Valiant's fate. (IDW Star Trek, Issue 1)