Hannah is most defined by the action of taking her own life. No matter what else we can say about her character, it always comes back to that. Hannah also defines other people by their actions; and because we only get Hannah's side of the story, these actions are often all we have to understanding the characters.
For example, Bryce Walker is defined as a bad guy by his wrist-grabbing and his possible rape of Jessica. Justin is defined as just plain mean by his rumor-spreading. Jenny is defined as inconsiderate by her failure to care that her drunk driving might have caused an accident. Hannah even comes to define herself by her actions – like her choice to have sex with Bryce.
Sex actually characterizes Hannah more for her classmates (the other characters) than for us as readers. Hannah doesn't take sex and love lightly, but there are rumors that she is sexually promiscuous. Although she works hard not to do anything to further that reputation, she's still defined by it. Even Clay, who has feelings for Hannah, defines her by her sexual reputation.