Agent Bill Harken was a Hyperadrenal Alpha, meaning that his body basically produced an overdose of adrenaline, making him a natural super soldier. Bill was extremely strong and fast. When he was "in the zone", he seemed incapable of feeling physical pain. However, the effects of his injuries came back to bite him once he calmed down.
The most obvious downside of his Alpha Ability was his temper. Bill had anger management issues, which led to his suspension from the FBI just before he joined the DCIS. Aside from that, his powers also took a toll on his physical health, especially on his heart.
Bill was married. He and his wife, Jeannie, never had any biological children, but they adopted a baby with Alpha Abilities.