the truth
Pheme, for all her bluster and bravado, feels incredibly insignificant. The fact that it was so easy for her mother to send her off, combined with the knowledge that she's got a father out there who she assumes knows about her but has never cared enough to look for her has left her with massive abandonment issues. That Agravaine hasn't kicked her out yet hasn't alleviated those whatsoever; her mother had her for thirteen years before booting her. This is why she clings to those people who act like they enjoy her presence, looking at them like a substitute family.
The advent of Charlie into her and Agravaine's life could cause some issues too, since she equates new kids with her family having no need for her. Tyr being around also complicates things, or will when Pheme realizes who he is. There will be resentment issues, especially because he seems cool and she's operating under the mistaken thought that he never wanted her.
In short, she's got crazier than average teenage girl problems.