User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-4649443-20121111201446

Piper's both Greek and Roman at the same time(just posting this so everyone knows).

The evidence:

Piper's mom said she is Venus and Aphrodite at the same time.

So actually, the Heroes are balanced:

Three Greeks:

Percy, Leo, Annabeth

Three Romans:

Jason, Hazel, Frank

One Greek and Roman amalgam:

Piper