Who is the antagonist?

Kathy seems to be facing a large number of antagonists and not one central opponent. First, Hailsham and the situation she is born into can be seen as a roadblock. The gaurdians sheltered her too much, so she didn't fully understand the world she's up against. But it doesn't stop there; Tommy could also be seen as an antagonist at times. He was a pain in his early years and hurt Kathy's reputation. But I wouldn't put him high on the list. Ruth, however, could be seen as a more frequent antagonist. While she was Kathy's best and most trusted friend, she goes to the extent of extreme embarassment in public. Kathy follows all of Ruth's rules. Kathy suffers through them constantly. But, there doesn't seem to be a single antagonist - the group of smaller challenges does pile up for Kathy, though.