There are two main characters in this story: Christine and the man who stalks her. We are not told much about any development in the stalker, so I will focus my answer on Christine.
At the beginning, Christine is a fairly ordinary young woman, who is neither shy nor particularly outgoing. She has the manners and grace to help an Asian man who claimed to be lost, and it is this encounter that changes everything for her.
After some time, he shows up at her house and behaves strangely. He then spends an entire summer writing Christine letters while she is working at a summer camp, and the fact that she does not answer any of these letters does not deter him. In an odd way, the more obsessive the man becomes, the more Christine's confidence grows. Although she does not admit it to herself at the time, she finds being irresistible to a man intoxicating.
When the police are eventually called, he is sent back to Montreal and later deported after he stalks another woman there.
After the excitement of being relentlessly pursued, Christine gradually loses the confidence that she gained from knowing that she was an object of obsession to her pursuer. Her life becomes very normal—she graduates college, gets a job and starts to put on weight.
In a nutshell, Christine strangely thrives on the attention that she receives from her stalker. After the attention goes away, she returns to her shell and lives a comparatively dull life.