Hadrian was born to a Dalmascan woman who had chosen, many times, to be unfaithful to her husband while he was away at war. Because of how long her husband was away on the border, she was able to get away with her infidelity. Ultimately she gave birth to him and gave him over to the slaves.
A slave with a young baby of her own took him so that he would not starve, but when he was old enough Hadrian was essentially cut loose and directed towards who they believed to be his father, a blood fighter - Hadrian would learn, at some point, that his father would have named him Declan, given the chance. He has never really felt close to that name though. He was put to work in the house as early as possible, in an effort to ensure he earned his keep.
Eventually he was sent to the Ludus - the son of a reasonably proven blood fighter, it made sense that the master of the house ultimately made that decision. He trained for some time before being allowed to make his debut - but he wasn't bad by any means. He was making a modest income for the house with his learned skills.
Hadrian had earned his brand. A disagreement between his father and the master of the house ended in his being sold as collateral damage. In the end, it isn't surprising - his father was more well established and bringing in more hessions overall.
He doesn't know much about the house that bought him yet, but he's apparently going to act as some kid's bodyguard. That shouldn't be too bad, right?