Helen of Troy was a woman from the ancient city of Troy, considered to be the most beautiful woman in the world.
Helen found herself upstaged when Supergirl arrived from the future, in an attempt to set her cousin Superman up with Helen. Helen's suitors all lost interest in her, as they marvelled at Supergirl's powers. As Supergirl's plan had caused too much trouble, Superman and Supergirl left that era. ("Superman's Super-Courtship!")
Prior to her visit to Helen's time, Supergirl had considered writing an essay about Helen, which would have involved traveling back through time to meet her. But she decided upon three other historical women instead. ("Supergirl's Three Time Trips!")