The character of John Andre appears, on the surface, to be something of a gallant. A suave, handsome, debonair British officer, he's someone to whom people, especially young women, are naturally drawn.
One such young woman is Sophia Calderwood, who engages in a flirtatious relationship with the Major after he comes to stays in her family's house in New York City. Sophia finds the young man's company most agreeable. As well as being unfailingly polite, he's also well-educated, able to speak French and German.
But despite his smooth exterior, Major Andre is not all he seems. In actual fact, he's a spy, involved in Benedict Arnold's treacherous plot to hand over West Point to the British. Sophia herself gets caught up in the plot, not least because of Andre's charms.
In due course, however, she puts her country first and exposes Andre as a spy. In doing so, she is making good on a pledge she once made that she would avenge the death of her brother William, who died aboard a British prison ship.
Andre, however, didn't make good on his. He promised that he would do everything he could to help William, but in the event, he reneged on his promise. It was this refusal to stay true to his word, more than anything, that reduced Major Andre in Sophia's estimation.