Lieutenant Lee Kelso was a navigator and engineering officer aboard the USS Enterprise in 2265 under the command of Captain James T. Kirk. Among his friends aboard ship was helmsman Gary Mitchell, whom Kelso referred to as "Mitch".
Kelso was on the bridge of the Enterprise when the ship encountered an object, later found to be the recorder marker of the SS Valiant. It was Kelso who alerted Kirk that the object was within tractor beam range and whom Kirk then had transport it aboard the ship.
While investigating the destruction of the Valiant, the Enterprise transited the galactic barrier and her warp engines were severely damaged. During the incident, Mitchell was seemingly injured and confined to sickbay, where Kelso later visited him during his coffee break. Mitchell, who, unbeknownst to the rest of the crew, had developed extraordinary psionic abilities from the encounter with the barrier, alerted Kelso that the points on the starship's starboard impulse packs were decaying. Kelso at first thought Mitchell to be joking, but the helmsman insisted. When Kelso investigated the matter, he found that the points on the packs were indeed burned out – something he had failed to notice when he checked over them prior to his discussion with Mitchell.
The Enterprise limped to Delta Vega where Kelso, a "talented thief" (as Scotty referred to him), cannibalized the equipment there to repair the damaged engines. Kelso was later murdered on Delta Vega by Gary Mitchell, who strangled the navigator using a cable from the equipment that Kelso had been working on. (TOS: "Where No Man Has Gone Before")
It is popular belief that Kelso was the helmsman of the Enterprise while Mitchell was the ship's navigator, as suggested by their positions at the conn. However, on-screen evidence suggests otherwise. Firstly, Mitchell's responsibilities aboard the Enterprise – neutralizing warp speed and piloting the ship, for example – are those of a helm officer. Secondly, when Mitchell is knocked out of his chair after being "zapped" by the energy barrier, Kirk yells out "helmsman!", an exclamation aimed at Mitchell. Third, Kelso himself was credited as "Navigator" in early cuts of the episode. In this early episode, the stations are reversed: Mitchell's post is the navigator's station and Kelso's the helm station. It is never explained – either on-screen or from behind the scenes – why their positions at the conn were reversed. Kelso wore an operations division beige Starfleet uniform, even though helmsmen and navigators usually ended up wearing the gold uniform of the command division in the rest of TOS. The division insignia on his assignment patch was the stylized globe symbol that would later be exclusively for sciences division personnel.
DC Comics' non-canon biography of Kelso in Who's Who in Star Trek 1 states he was born in San Francisco, California, and studied at the Palo Alto Technical Institute and at Starfleet Academy, where he was a classmate of James Kirk.