Myrtle is a woman who is looking for a way to improve her situation. She wants to move up in the society circle. She is married to George, but she knows he will never be able to give her the life she wants. While she is on the train into the city to see her sister one day, she spots Tom. The two are instantly attracted to each other and from that moment on they become lovers.
Myrtle sees Tom as her way to increase her standing. Tom only sees her as an object that he wants. Tom isn't going to leave Daisy for Myrtle. Myrtle is not the type of woman Tom would ever be with in the society circles, he only uses her for his own pleasure. Myrtle actually thinks that Tom loves her and believes that he will be with her one day. Neither Tom or Myrtle think about the consequences their affair will have, not only on the other people in their lives, but to themselves, as well. Myrtle is an opportunistic woman who goes after what she wants. In all seriousness, Tom and Myrtle are a lot alike. They both care about how society sees them, and they want to have the best. The two of them are actually perfect for one another, but Tom's marriage to Daisy has solidified his place in society and he will do nothing to jeopardize that. He thinks that he can keep his affair quiet and no one will ever find out about it.
If you look deeper at the character of Myrtle, one might feel sorry for her. She thinks that Tom loves her, when in fact he could care less about her. Myrtle is an unhappy woman, married to a man she doesn't love and with Tom, she gets a taste of the life she wants. She has no idea of how badly and deadly this infatuation will be for her.