Not much is known about Kenny Ackerman, aside from the fact that Levi is his nephew, and that he did take him under his wing for a short time, after Kenny's sister -- Levi's mother -- had died. The surname Ackerman is one seen as representative of hated people, or at least misunderstood, like the Asian race in this series. It is not clear on what basis Kenny and Mikasa Ackerman are related, if at all. Kenny was the captain of the Military Police Brigade, as well as the true royal family, and in turn, Uri and Rod Reiss' most trusted associate. It is because of his association to the royal family, that he has committed atrocities without consequences, and his main role, though short, was to prevent the Special Operations Squad from retrieving Eren Yeager and Historia Reiss.
It was later revealed that he wanted Eren's Coordinate ability for himself.