As for why Brelia is in the arena, originally she was a volunteer healer for the winners of each bout, so that they could fight in whatever their next round would be in top form. To this end she'd also provide psychological help. After doing this a while, she sometimes had gladiators who leave her healing hands and go off to die in the next round. This happened one too many times, and she got thrown into the arena as a gladiator herself after being accused of sabotaging the gladiators under her care.