Ares gives boons that inflict Doom, create Blade Rifts, or increase the damage of your attacks.
If given Nectar, Ares will give you Blood-Filled Vial.
Ares' affinity gauge is maxed out at 7 hearts.
Ares' favor requires completing the War-God's Bloodlust prophecy and slaying at minimum 10,000 enemies total. The affinity gauge will not immediately unlock upon completing the prophecy, but rather from a dialogue with Ares a few encounters later.