Atenra is, as far as most people know, the second-born son of the Lancaster family. However, his biological father is a blood fighter slave by the name of Gannicus though he does not know this. In fact he may be one of maybe three people who don't, or didn't, within the House.
The firstborn son is Canius, believed by Atenra to be his eldest brother. Canius is biologically Titus' son, the only one of the four, and he was doted on by Titus. Atenra's youngest brother is also his only full-sibling, and his name is Sepheres.
Atenra was never really under any illusions that Titus was his father - his skin tone is too dark, too obviously different from both his mother and Titus. It was obvious some third party had been involved, even when he was young. Titus never really treated him like a son, either. He was not generally directly abusive - Atenra suspects mostly because of Victoria's influence. He does bear the Lancaster family name. But he's never really had illusions that it belonged to him, or that Titus wanted him to have it.
Atenra was left mostly to Victoria's care, which suited him well enough. She ensured he and Sepheres were taught to read, among other things. Atenra may have been sheltered from Titus' wrath, but he was not unaware of what his brother was put through at the hands of their 'father' and Canius. That is, unfortunately, reality. They were forbidden from ever going near the blood fighters and that is one of the rules that Atenra never risked bending or breaking.
Things went on like this for a time, and the boys grew up. Reggie was away at war a lot, and Canius was spoiled. Things remained stable for a good long while, and then Umbrius was killed in a mysterious fire in the blood fighter's quarters. With Umbrius moved out of the way, things shifted, and Aurelia went missing shortly thereafter.
The precarious balance that had kept things together for so long was shifting, and before Atenra had time to really understand what it meant, things drastically changed again. Canius had done something terrible, the last time he and Victoria had fought. Atenra had his misgivings, it wasn't as if there weren't whispers that warned precisely what Canius' proclivities were.
And then one of the blood fighters dared to argue with Titus, and was sold for it. Gannicus. Atenra isn't familiar with the blood fighters, not really. But after that, something seemed to break, for his mother. And in the end, in the dark of night, she threw herself off of the cliff that the training grounds sat upon.
Atenra mourned her loss. She was a good woman, and she had done the best she could, as a mother. But there wasn't much time. When she died, her protection died with her. And without that, Sepheres was sold shortly thereafter.
It wasn't long after Sepheres was sold that Atenra was sent, by Reggie, to stay with Livia at House Asheron. Regillus thought it was safer - he was right, to be fair. But Atenra can't help but feel like it feels like running away.
Maybe there's no shame in that though. Still, he wishes they had some idea of where Sepheres was, and if he was okay.