Carter Winston was the alias assumed by a Vendorian as part of a subterfuge plot. Prior to assuming this false identity, this Vendorian tended to the real Carter Winston after his trading vessel crash landed on the planet Vendor, until Winston's death almost a year later in 2265.
In 2269, this Vendorian began working for the Romulans, in the guise of Carter Winston. As part of the ruse, the impostor set Winston's craft adrift near the Romulan Neutral Zone, where it was eventually discovered by the USS Enterprise.
After successfully convincing the crew he was Winston by producing the Human's identity tape and undergoing an examination by Doctor McCoy, he "reunited" with Winston's fiancée, Anne Nored, telling her that he'd changed and couldn't ever marry her.