In my story, the protagonist has a friend with benefits kind of relationship and he falls in love, but she doesn't love him back. At some point she sleeps with another guy, that ends their relationship and she exits the story. The plot then focus on him trying to get over it and becoming a new person through the experience.
When I started writing drafts, I noticed I was making her the antagonist, depicting him as an evil person that doesn't care about my MC's feelings. Though I still think she doesn't care about him, I don't think she is evil anymore. The relationship was about sex, it was transactional, and she is not a bad person for not loving him back. This kind of shit happens.
Still, he is the protagonist, and I want the reader to care about him. How can I deal with this, in a way that the reader cares about him, but doesn't hate her? How can I make her a "force of nature" within the plot?
Source: reddit post