John Henry "Doc" Holliday was a hot-tempered 19th century gambler and gunfighter from Earth's Ancient West. Prior to gambling and gunslinging, he was a trained dentist. While traveling the Midwest, he met and befriended legendary lawman 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")
In the novelization of the episode, Doc Holliday is far more friendly towards McCoy 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.