Regarding Bulls (note)
To actually become a Bull, the applicants are hand-picked by the important members of this faction. Oftentimes with the client they'll be working for already in mind. The training they have to finish until they are ready to get sent out if the longest and most thorough inside the FCC and normally lasts for 14 years. They don't have large classes, like the Hunters or Shields have, and more than once there was only one suitable trainee for a year. Since their work mostly consists of keeping the client secure, they concentrate fully on achieving just that. And since some Bulls work for whole families, some of them kept more than one generation safe.
So to ensure that they train the perfect bodyguard or safe-keeper, they start picking out the children as young as possible. Sometimes those children get sent there by their parents, sometimes they're already orphans, and sometimes a slavetrader got a hold of a really special kid. Once they're picked, the are certain to lead a fulfilling life, never going hungry, never without a place to sleep and never missing any sort of luxury they may need. Some may argue that the Bulls are the slaves of the client that bought them, but their freedom is never discussed upon. The client may pick the Bull they want, but the contract is finalized as soon as the respective warrior finished their training.