When you and Hercules enter Mount Olympus, Hercules sees a small snake and attempts to get rid of it. As it turns out, the snake was actually Medusa, who appeared and promptly turned Hercules to stone.
Percy Jackson and the Lightning Thief Ad
Medusa also makes an appearance in the Percy Jackson and the Lightning Thief ad as a major antagonist, except here, she has red rays of light radiating from her eyes.
Despite being a enemy, she doesn't stand in the player's way at all, she simply turns Hercules into stone out of annoyance.
Because of her minor role and very brief appearance, there is no known way to customize her (excluding the snake hair that can be bought in-store and the redeemable costume that was loosely based on her design)