Hercules is a fat cat who fell in love with Luna, saving her from several alleycats. He is also the seventh rainbow crystal carrier (though his owner, a little girl, was initially mistaken for the crystal carrier), and therefore has the spirit of Bakene, one of the Seven Shadows, inside him. Zoycite chased after Hercules and Luna (being attacked by rats in the process), and eventually took out his violet crystal, and transformed him into Bakene. A small part of his influence still lived on inside Bakene, where he saved Luna from falling. Hercules was healed by Sailor Moon and returned to his owner.
In Episode 37, Tuxedo Unmasked, Hercules was recaptured inside a crystal by the brainwashed Prince Darien, who was trying to capture all the crystal carriers to use the Seven Shadows' power (which was not destroyed when the shadows were) to awaken an invincible shadow monster, but was released when Sailor Mercury broke the crystal.