Lieutenant Stiles was a male Human Starfleet officer who lived during the mid-23rd century. He served in the command division aboard the USS Enterprise under Captain James T. Kirk, during Kirk's historic five-year mission. During the Neutral Zone Incursion of 2266, Stiles was serving as the ship's navigator.
Stiles evidently came from a family with a long history of space service; he mentioned a captain, two commanders, and several junior officers who all served during the Earth-Romulan War – and who were all lost in that war. These family losses left him with an inherent hatred of Romulans that was perhaps greater than that of most other officers.
Though Stiles' first assignment was weapons control prior to his assignment as navigator aboard the Enterprise when the ship responded to the destruction of several Earth Outpost Stations on the Federation side of the Romulan Neutral Zone. His family history made him suspicious that the outposts' attacker was Romulan – a suspicion that turned out to be accurate, as the perpetrator was a Romulan Bird-of-Prey.
When it was learned the Romulans were externally almost identical to Vulcans, Stiles transferred his hatred to Spock, making a number of thinly veiled insinuations that eventually drew the ire of Captain James T. Kirk; Kirk remarked that bigotry had no place on the bridge.
During battle with the Romulan ship, close-range detonation of a nuclear device left the forward phaser control room manned only by Specialist Robert Tomlinson, with no standby crew. Given Stiles' familiarity with weapons control, he left the bridge to assist Tomlinson. When the phaser coolant line developed a fault, Stiles and Tomlinson were overcome before they could escape. Fortunately, Spock was nearby and was able to rescue Stiles before the gas could kill him. Stiles gratefully accepted that Spock had risked his own life to save his, and that he wouldn't be alive without Spock having done so. (TOS: "Balance of Terror")
Stiles was portrayed by Paul Comi.
Stiles' first name is not established on-screen but was given as "Andrew" in a cast list that was written for the script of "Balance of Terror" (despite the name not being used at all in the main body of the script). He was also listed as having that first name in reference works Star Trek Concordance and The Star Trek Compendium, the comic book sourcebook Who's Who in Star Trek, and the novel The Higher Frontier. For a time it was also noted at StarTrek.com.
The revised final draft script for "Balance of Terror" fleshed out Stiles' suspicion that Romulan spies might have been aboard the Enterprise. A line written for Commander Hansen indicated that the Bird-of-Prey incorporated "starship design" elements stolen from Earth by spies or traitors. 
In the Bantam TOS novelization of "Balance of Terror" (printed in Star Trek 1), author James Blish referred to Stiles as second officer (even though he seemed to be outranked by Montgomery Scott in the canon episodic version of the story). Blish's treatment also listed Stiles as a fatality, removing the premise that he was saved by Spock, as seen in the episode.
Several other non-canon works mentioned Stiles. He appeared in the Pocket TOS novel Constitution. The comic "Debt of Honor" established that he became a security officer in his later career, with the rank of commander. The Pocket TNG novel Red Sector stated that he was a commander in security at retirement and indicated he might still be alive by the 2350s, but also listed his name as "John Stiles". Starfleet: Year One had an "Aaron Stiles", his possible ancestor, as a main character.