User blog comment:It-Freaking-Melts~11!!/New Characters for Sons of Neptune Decoded/@comment-3146930-20110906001306

Ok, so evidence to support Mars:
 * He is a mastery of weaponry
 * He uses a spear (Ares normally has a spear in mythology)
 * The Wolf is sacred to Mars
 * His parent is suppose to be surprising, so if he carries a bow (Apollo), Mars would be a surprise
 * Mars is one of the Olympian Week gods
 * A book summary says that death (Thanatos) has been captures and Percy, Frank, and Hazel must save him. In mythology, Ares was the one that freed him from his chains and patterns tend to repeat themselves in the series. So unless Hazel is the child of Mars, it has to be Frank