He will be able to ally with one of the various powerful factions who are vying for control of The City, which will affect not only Aiden, but everyone else in The City. He's looking for his sister, from whom he was separated after they were both subjected to scientific experiments as children.
Protagonist Aiden is a runner in The City, someone who runs errands and handles odd jobs for the human citizens. Source: Techland (Image credit: Source: Techland)