Forenote: With all the writing I am doing on this lately, I think I'll go consolidate this into one place sooner or later.
Now: Esna's staff and crews are a combination of many things, with a strict hierarchy in place between three distinct castes - a reflection, in many ways, of the Empire he grew up in.
At the top are the "Determined" - this caste forms the crew on his core vessels, officers on less important ones, and the overseers and chiefs of his facilities. They are people who show religious, social, or personal conviction for joining his forces, and can thus be relied on to 'do the right thing' for more reasons than pure money. While, of course, the enticements of good treatment, decent pay, and care for family are a draw to many, it is only those who choose to fight for a higher cause that fit into this first group. Notably, Esna is perfectly happy to abuse a grey zone in the laws regarding Holders and starships: Although a Holder is forbidden from raising his own space fleet, the rules regarding forming special deals with certain "private military contractors" are rather more nebulous. So what if the contractor happens to be solely owned and financed by a certain capsuleer... in this way, Esna is able to funnel commoner recruits and contracted slaves into his crews to augment those who volunteer on their own. In the case of slaves in this 'caste', duty on a combat vessel is optional, but explained as a likely faster track to freedom or a comfier position.
The second slot are true mercenaries: Those whose only attractions to service with Esna are glory, pay, and the thrill of fighting under a capsuleer. These are also funneled through a private military contractor Esna sponsors for the sake of simply keeping all the recruits in one pool and one system. Unless they happen to start showing signs of developing a sense of faith or conscience, they are generally end up assigned to non-critical positions in facilities and basic duties aboard some vessels.
Finally, a small number of slaves are funneled into his crew as penitents. Captured pirate crews, Blood Raiders, or terrorists who have already committed crimes and whose hatred of the Amarr is judged to be so great they will surely strike again, these are used as a resource for crewing the most dangerous vessels - ships whose lifespans following undocking can be measured in hours. Most do not survive, but the ones who do somehow manage to live through enough 'runs' are eventually transferred to some other duty where they will serve out the remainder of their lives. Esna doesn't like to talk about these.