Your sister, your responsibility.
Bellamy was too young to really understand the words the first time they were spoken to him—too young to realize why his father changed so much when he remarried, too young to grasp the concept of something so powerful it could consume a man’s soul.
But he understood enough; he understood don’t let her cry or they’ll find her, and if they find her they’ll kill her. The why didn’t matter; what mattered was this tiny thing squalling herself bright red, her tiny fist clenched around his finger like he was the only stable thing in her world.
The first time Octavia’s eyes went black, Bellamy thought she was dying. He rushed to her, intent on helping, but their mother pushed him back. Octavia’s power released without target, and her slight six-year-old frame slumped to the ground as she lost consciousness.
It was the most terrified he’s ever been in his life. Even now, even with Mord-Sith and magically mutated beasts and John Murphy, with what he did to get himself through the Boundary with the hundred, what he has to do now to cover it up; none of it can compare to that first time he watched his little sister pass out and knew in his bones that he had failed to protect her.
His mother—for she’d always been Mother to him, not Stepmother or Aurora or Octavia’s Mom—sat down with him, after making sure Octavia was curled up safe and comfortable in her hidden alcove. She told him how special Octavia was, how important. How she carried remnants of the two most powerful bloodlines in history, and how that made her a target. No one could ever know about her, or her powers.
Bellamy spent every waking moment since doing everything he could to keep Octavia’s secret, to keep her safe—until one stupid mistake exposed her, one colossally ill-considered moment when his desire to make her happy ended up condemning her instead.
If only she’d been able to control her power—if only he’d realized she couldn’t. Having a second child was a crime, yes—with limited resources and no way through the Boundary, there was only so much the wizards and artificers could do to provide for everyone—but they might have gotten out of it on the technicality that Octavia was their mother’s first child, even if she’d been their father’s second.
But Octavia got scared, and the guardsman pushed, and it was dumb luck that Bellamy managed to pull him away just seconds shy of being confessed. Not that it mattered; her power was exposed, she was exposed, and there was nothing Bellamy could do to stop them from arresting her.
The next day, they sent his mother into the Boundary. A cruel form of execution—death by madness or spirits or hunger, whichever happened to get to her first—but one that spared the Council from having to face what they were doing. No body, no cleanup, no consequences.
When Shumway put a dragon’s breath pistol in his hand, Bellamy almost didn’t need convincing. It was Jaha and his Council that killed his mother, that were now sending his sister off to die—it was almost poetic that he be the one to end Jaha’s life.
He would do it all over again, too. No matter what happens from here on out, he did what he had to do to protect his sister. He always will. It’s why he encouraged the other kids to smash their blood crystals, to cut off any chance of communication with Aydindril; why he’s now watching fire lick at the pages of the journey book he stole from an unconscious girl in the wreckage.
If the Council gets through the Boundary, they’ll kill him. If he dies, he can’t protect Octavia. Bellamy won’t let that happen.
His sister, his responsibility.