The Spirit Gods are immortal supernatural entities in Ryse: Son of Rome.
They are simply known as Summer and Winter, after the season that they correspond to. Winter also masquerades as a mortal and goes by the name Aquilo. Although the two are immortal spirits and therefore genderless, Summer chooses the form of a woman while Aquilo appears in the form of a man.
The two are locked in a sort of "game" to them, in which they play with the fate of humanity and use individuals as pawns to accomplish their ends. While the game has rules, Aquilo tends to disregard them.
Summer is benevolent and supports great empires and civilization, believing that order and progress are the best thing for mankind. In contrast, Aquilo is for the most part malevolent or at the very least has total disregard for the well being of mortals, and supports primitive tribes and peoples, believing that all progress is bad and dangerous. Aquilo aims to destroy civilization and drive humanity back to a pre-stone age state, and works to undermine the Roman Empire in order to accomplish his goal.