Aokiji, aka, Kuzan, was a former Marine Admiral. He, as we know, was elected by most of the Marines to be the next Fleet Admiral.
But, the World Government wanted Akainu to be the Fleet Admiral. These two battled with each other at Punk Hazard for 10 days, and Aokiji was defeated by Akainu. Later, Aokiji resigned from the Navy and also became a member of the Blackbeard Pirates.