His name was Jaed. As a slave he was treated mostly well, with the exception of his direct master. He was owned by a pure blood Sith family which had the mother and father, the father being the head of the family and a Sith Lord, two daughters and a son. The younger daughter was smaller than him, the son was his age and the older daughter was two years older than him. He was born into slavery, into this household by his also slave mother. He and his mother, both human, were two of the many slaves the family owned. His mother was very pretty and the father had enormous affection for her, she was his main concubine. Even though the father was Sith and had little to no respect for slaves, he treated his mother well.
There were rumours that Jaed was his child, so from a very young age Jaed had no real friends among the slaves and hung out with the family's kids. The oldest daughter fell in love with him and they formed a sort of relationship. They quickly came in a romantic, sexual relationship which they kept for little more than a year. He lost his virginity to her and she to him. He was in love with her for the better part of their time together. Because slaves were considered lesser beings, they kept their relationship a secret from everybody. At some point, the son found out, he caught them making love in a barn, and he caused a big scene. The father scolded and beat Jaed in a public display of torture. That's when Jaed unleashed his power. At some point during his flocking, the father was beating him with a whip (that's how he got the huge scar that covers half his face) and Jaed broke the chains that restricted him, electrocuted the father and the guards. He killed the guards and hurt the father during that process. He was eventually subdued by the father, but he gave a big fight until then.
He was imprisoned and the entire household, slaves and the family alike, believed that Jaed had died by the father's hand. Instead, the father sent Jaed away, to the slave pits of the Empire. That's where Jaed decided to change his name. He introduced himself as Occlus. He wanted a name with some significance, vaguely threatening, vaguely ominous sounding.
At the slave pits he started causing scenes with his overseers, till a Sith Lord who was overseeing the mines met him and felt his power. The Sith Lord was sent to recruit slaves with force sensitivity as acolytes. The Lord took Occlus to Korriban by force at the start, but then described to him what being an acolyte means. Occlus decided that this was his destiny, to be Sith himself, and so our story starts.
He has bright blond hair, a very pretty face, even with the scar his father gave him, and he is very well built. He is loyal, due to his slave years, has an honour code, due to his friction with the Sith family, he is vengeful, he takes insult to people who don't respect his power, but gives big credit to those that do. He has good manners given he was brought up around royalty and he carries himself differently than his peers. As he becomes more powerful, his presence often demands respect.
He has a crash on Zash, he really likes her and is happy to do her bidding. He would indeed have killed both Harkun and Ffon if Zash hadn't intervened, even though he started out wanting to please Harkun.
He is not bloodthirsty. When he meets resistance, he threatens first, but if the resistance persists, he doesn't hesitate to make good on his threat. In general, he kills those that betray him, or severely threaten his position.
He desires to love and be loved. He fell in love very early in his life with his master's daughter and they had a very passionate relationship. She was his first lover and he fell hard, he wanted to be with her forever. He was forcibly separated from her, he didn't even get to say goodbye. He has been chasing that feeling ever since.