First off crew can sometimes be hard to come by. Unlike many things in the game, they can't be sold, bought, traded, or stored anywhere. They will only serve on your ship if you hire them, rescue them, or accomplish some kind of task for them.
 
They also expect to be treated and paid well. If they get unhappy enough they will refuse to work and eventually will even leave your sorry butt. So how does this work? Well first they expect to be paid every once in a while, so they slowly get upset between payments. They also really don't care for being hurt or killed, imagine that. Each individual is also from one of the races in the game. Do something bad to their race and they are going to get upset. Do something nice for their race and it will make them a bit happier. Actually paying them is the biggest positive contributor to their happiness of course. It is your choice when and how often to pay them.
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