John Henry "Doc" Holliday was a hot-tempered 19th century gambler and gunslinger from Earth's Ancient West. Prior to gambling and gunslinging, he was a trained dentist. While traveling the Midwest, he met and befriended legendary marshal Wyatt Earp.
In 2268, the Melkot created a replica of Holliday as part of a bizarre, inaccurate depiction of the events leading to the gunfight. This illusion was intended to lead to the execution of Captain James T. Kirk and his landing party. (TOS: "Spectre of the Gun")
Doc Holliday was played by actor Sam Gilman. While Gilman was 53 when he made the episode, Holliday was only 30 when the shoot-out took place.
In a line cut from the 9 May 1968 "final draft" of the script, when Sylvia first mentioned "Doc Holliday", Leonard McCoy asked if "[h]e's a doctor?", she replied that he was a "[l]ousy tooth puller. He's no better'n the Earps."
The novelization of the episode, which is actually based on the barber shop scene in the same script that was later entirely rewritten, Doc Holliday is far more friendly towards McCoy. This became evident after McCoy mentions he was born in Georgia (the same as Holliday) and recognizes Holliday's symptoms of tuberculosis and offers to cure him. Holliday extends an offer of friendship as well, stating he will spare McCoy's life if McCoy betrays Kirk and the others and joins Holliday and the Earps. McCoy declines, but is graciously offered the needed tranquilizer drugs by Holliday who nevertheless ends their meeting with a caution that he will still shoot straight at 5 o'clock.