Harrison was a 23rd century Starfleet technician (first class) serving aboard the starship USS Enterprise under command of Captain James T. Kirk. During Kirk's five-year mission, Harrison played a part of many of the ship's missions, and eventually reached the grade of lieutenant.
Harrison, wearing a blue sciences division Starfleet uniform, was part of the bridge staff during a 2266 encounter with the First Federation ship Fesarius. Harrison was one of many bridge crewpeople who were violently ejected from their seats when Enterprise was forced to destroy Balok's radiation-emitting space buoy. (TOS: "The Corbomite Maneuver")
Harrison was working in the biopsy lab in 2266 when Enterprise personnel were afflicted with an intoxication from Psi 2000. Upon McCoy's request for Harrison's report, an unnamed crewperson began laughing hysterically over the speaker. Harrison was later at his post in sickbay (wearing a sciences blue jumpsuit uniform) helping to restrain Hikaru Sulu while McCoy tested antidote serums. (TOS: "The Naked Time")
Harrison was part of a security team that responded to a call of trouble at the ship's theater. Lenore Karidian stole Harrison's phaser to hold Enterprise crewpeople at bay. She accidentally discharged the weapon and killed her father, Kodos. (TOS: "The Conscience of the King")
When Khan Noonien Singh attempted to take over the ship in 2267, Harrison wore a red operations division uniform while among the crew trapped on the bridge. Kirk's captain's log noted commendations for numerous personnel, including Technician Harrison, who lost consciousness from oxygen deprivation. (TOS: "Space Seed")
Lieutenant Harrison, wearing a gold command division uniform, was among Starfleet personnel that beamed down to Omicron Ceti III in 2267. When sub-sonic vibrations were used to irritate the Omicron spore-possessed crewmembers on that planet, Harrison was overcome by the effect and started fighting with Lieutenant Kelowitz and another crewmember. (TOS: "This Side of Paradise")
Harrison was played by regular TOS extra Ron Veto. Harrison's name was mentioned in "Space Seed" as the camera panned over Ron Veto. In this scene the name Technician 1st Class Thule was mentioned as the camera panned over actor Eddie Paskey, who generally played Lt. Leslie, and he can be clearly seen wearing his lieutenant braids. Also the camera has moved past actor Blaisdell Makee, who played Spinelli, by the time his name was mentioned. There is the possibility that Thule was the name intended for Ron Veto, who had no braids on his costume, since the name also refers to an Eskimo culture, indicating a possible Inuit background (the actor was actually of Hawaiian descent). This would be consistent with other character names in the series that are used to describe place of international origin. The other use of his name, in "The Naked Time", did not show Veto when the reference was made, and featured a communications response from Harrison that was dubbed over laughter voiced by John Bellah. Seeing as Veto was seen later in that episode, unaffected, its likely that the voice wasn't Harrison's at all, but rather Bellah's unnamed crewman character intercepting his call. Veto is one of only a few to wear the colors of all three divisions (sciences blue, operations red and command gold) in his various appearances.