Ares is his Greek name. Mars is his Roman name.
In the film he has a very minor role in which he only appears in minor cameos.
In the beginning of the film he is present for the party celebrating the arrival of Zeus and Hera's newborn child Hercules. Ares, along with the other God,s was sent to find Hercules, but it was too late as the baby had already been turned into a mortal and was adopted by a mortal family. Eighteen years later, he is seen imprisoned with the other Gods after they lost against Hades and the Titans. Luckily he and the other Gods are freed by Hercules.
On Sora's TeamEdit
Ares was happy to meet Sora and fight against the Heartless, Subspacers, and Nobodies. When the team had to leave as their mission was over, he almost had a chance to join their journey but was to late, and decided to wait as they come back.