User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-170.235.245.51-20130606171516/@comment-3146930-20130606212026

Ya, the form they are in really depends on who is thinking about them at the time. In a Roman camp, Ares would always appear as Mars, even if one person sees him as Ares. With Bacchus, there was almost an even number of Greek and Roman demigods. The moment Percy started thinking of Dionysus, Bacchus began to change back, but stayed in his Roman form.