I've noticed that a lot of people don't like Piper for one reason or another, but I just want to point out a few things. First off, during most of the first half of The Lost Hero, she was told that a giant would kill her father unless she helped him capture/kill Leo and Jason. She doesn't appear to be the kind of person that would betray someone easily so this is a big deal. Imagine trying to pick between your friends and your family, and one of them has to die to save the other. It isn't a pleasant feeling
There is also her relationship with Jason. Even if Jason only appeared on the bus that morning, she was implanted with almost 3 months (I'm going by memory here so it could be more/less) of false memories of them being bf/gf. Then Annabeth points out the memories are fake which completely destroys her emotionally. Even worse, she is going on a quest with him and has to figure out how much of her feelings for him are her or the mist, as in how much of her feelings are real? She also has to figure this out while she is leading him into a trap that could get him killed.
While I don't like Piper as much as Annabeth as a female lead, I dont dislike her or anything and she breaks the mold of the Aphrodite camper only caring about looks. I like her as she has a lot of emotional problems to deal with and possibly has the most development from going on the quest.