A woman Kirk loved years before, recreated on the Shore Leave planet in Shore Leave.
Ruth is a woman James T. Kirk knew and loved earlier in his life, before his Starfleet career pulled them apart. Kirk has not seen her in years, though the memory of their relationship has stayed with him.
In Shore Leave, the amusement planet's technology recreates Ruth exactly as Kirk remembers her, and she appears to him among the trees. The reunion is one of several of the crew's memories and wishes made real. Kirk spends his remaining leave with her once the Caretaker explains the planet's purpose. She was played by Shirley Bonne.