Although largely hidden away in the marketing for the movie, Ares is indeed the main villain of the movie. It's an interesting decision, because for the majority of the movie it's only Diana who believes in the God's existence, at first on a more spiritual level before the realisation he has psychically manifested himself on Earth.
That leads to a reveal that is supposed to be shocking, but was actually spoilt some time ago. Even if you don't know going in who Ares is, it's probably not that hard to figure out.
What's harder to figure out are his motives, since there's very little screen time really given to the character due to making him a 'reveal'. The final battle between him and Diana is one of the movie's weakest parts, where it reverts most to type in terms of the DCEU and indeed a lot of Marvel and other comic book movies, a big, unnecessary CGI fest.
David Thewlis is usually an entertaining presence, but he's largely wasted here since he can't do too much with the character before it then transcends into its final, more Godly form.