Miss Elizabeth D'Amido, or Betsy for short, is a very pretty young lady who embarks upon a brief fling with Adrian Smith. Inevitably, this wreaks havoc on his marriage to Eva, who knows exactly what's been going on.
When Betsy first meets Adrian, she openly tells him that she fell in love with him the moment she first saw him. Furthermore, she tells him that she's seen his play four times. This would appear to indicate that Betsy is somewhat rash and impulsive, someone used to expressing herself freely without being contradicted. As is often the case in a Fitzgerald story, we're introduced to a spoiled member of elite society, someone who feels entitled to play with other people's emotions, utterly heedless of the consequences.
And play with them she does! It isn't before long that Betsy's managed to wrap Adrian around her little finger, flirting with him, playing deck tennis with him, and disappearing with him at regular intervals to snatch stolen kisses far from Eva's prying eyes.
Once Eva gets wind of Adrian's infidelity it seems that the Smiths' seven-year marriage is at an end. But thanks to two large crashing waves that bear down on their ship at an opportune moment, Adrian is able to convince Eva that everything that happened on that fateful voyage was just a nightmare.
By that, he simply means that he's moved on and wants Eva to follow suit—which is why he promises to buy Eva a set of pearls by way of a peace offering. So although Betsy D'Amido generates a lot of turmoil in the Smiths' marriage and comes close to wrecking it completely, she is ultimately unable to steal Adrian away from Eve.