Ruth was a highly sophisticated robot copy of a former lover of James T. Kirk, created by the Shore Leave Planet upon reading Kirk's thoughts of her.
The first time she appeared, Kirk was distracted by other problems, and could not spend time with her. After his discussion with the Keeper, Kirk planned to beam back to the USS Enterprise, but Ruth appeared again, and this convinced him to remain on the planet for a day or two. (TOS: "Shore Leave")
Ruth, or rather, her facsimile was played by Shirley Bonne.
The character was ultimately not cast as she was described in the final draft of the script, which defined her as "A truly beautiful girl with long black hair." It was, though, accurate in describing, "Her whole aspect is gentle, warm, welcoming."