You can probably fall back on a lot of tropes that work for cop show/movies. Just a few ideas off the top of my head:
-Kid of a guard trying to live up to the family expectations.
-Poor, unconnected kid trying to escape that life by joining the watch (steady pay without needing an education/apprenticeship).
-Rebellious son of a career criminal joining "the other side".
-Low-scale corrupt opportunist who's softened up and come to feel a sense of camaraderie with his fellow watchmen.
-Lost a loved one to crime (gang violence, serial killer, etc.) and feels duty-bound to stop it from happening to anyone else.
-Rescued by the watch as a kid, now hero-worships them and wants to be one.
