Sylvia was a creation of the Melkotians, an illusory Human woman who was part of a scenario devised for the purpose of executing James T. Kirk and his landing party in the year 2268. (TOS episode: "Spectre of the Gun", TOS - Star Trek 3 novelization: The Last Gunfight)
Sylvia became romantically involved with Pavel Chekov from the Enterprise.
When Chekov interfered when Earl Wyatt tried to take her away Sylvia witnessed Wyatt shoot Chekov to his death.
Her deafening scream alerted the rest of the landing party, who came running to the scene. McCoy told Kirk there was unfortunately there was nothing he could do. (TOS episode: "Spectre of the Gun", TOS - Star Trek 3 novelization: The Last Gunfight)