I love Patricia Highsmith, and the Ripliad in particular, but I find this the least effective of the series, mainly because of the boy of the title, Frank Pierson, whom I find annoying. To be fair he's only sixteen, and behaves like it. Passions overwhelm him, and he is enormously self-centred. Being the pampered son of a millionaire doesn't help him I suppose.
Quite a few of Highsmith's characters share similar characteristics to Frank - they're quite weak, passive-aggressive types, prone to obsessing, particularly about other people. A notable example is Ray Garrett in Those Who Walk Away. I think this probably reflects Highsmith's personality, which was shaped by her cruel and domineering mother.
I think Pat wanted to be carefree, elegant and amoral, like Tom Ripley (and his wife Heloise), but she was too damaged to achieve that. She longed for someone like Tom or Heloise to love her, which partly explains Tom's patience and dedication towards the brat. Other explanations being Tom sees something of his own awkward and difficult youth in the boy, (Tom's not a complete psychopath), and the sexual attraction.