Zoë.
As a reader, was I the only one that got slightly annoyed by her nearing the end of the Titan's Curse? Sure, she had a good back story, which many Percy Jackson characters lack, but at the end, she did start to test the fandom of Percy Jackson.
If you really think about her backstory in of itself, it was one of the best Rick created for a Percy Jackson character. And yes, I understand that "demigods" are supposed to be children who live normals lives then discover thier godly parent by monsters chasing after them, however, it's rare we see a change in backstory from any PJ Character.
Aside from that, I also like the incoorperation Rick made with Zoë and Percy, however it was a shame to see Zoë cut off from the rest of the world, where there was so much potentional to build on Zoë and Percy's relationship, which sometimes Uncle Rick refuses to do .
In the Love aspect, the emotional side of Zoë was perfectly balanced due to her being a Hunter of Artemis. The archtypical "fall and redemtion" story played critical in Zoë's love story, the little one she had.
A good girl, cut off before her time.