The first time she marks him, she's just a little girl and she can't understand the low noise he makes as the knife digs into his flesh. Their mother isn't there, she's gone and she'll stay gone until morning, but he adrenaline still races through her blood, the knowledge that if they were to be caught their mother would be angry consuming her.
"Does it hurt?" she whispers.
"Yeah," he breaths, blinking hard at the ceiling.
"Should I stop?" she says.
"No, please don't," he murmurs.
She draws a ring around his upper arm, digging the knife deep enough to leave a scar when it heals, and Bellamy cries out into the silence of their room, screwing his eyes closed. It hurts to hurt him, but at the same time she knows he doesn't show his hurts often, he keeps them tight inside and he shares himself in ways she can't understand yet, and she wants to help him.
When he cries out into the night, he's releasing something, and whatever it is... it needs to be gone.
The second time, they're older. She's thirteen and he's seventeen. He's a guard cadet and he's started working hard, not that he didn't work hard before, but now he's working a job and it's all she can do to help him when he makes it home in one piece.
He says it's safe, but when he describes his job it doesn't seem that way: guarding from criminals, protecting people. It sounds like ways she could lose him.
She asked her mother once, "Mom, what happens if Bell dies? Would I be allowed to live then?"
Her mother nodded, distracted, not caring maybe, and waved a hand. "One child is the law," she said. "They'd let you."
Her blood had iced over.
She didn't want Bellamy to die.
When they're lying together later, their mother long gone into the night, she swirls a circle on his chest.
"What's that?"
"A circle."
He pauses, quiet a moment. "Like an O?"
"Yeah." She digs the knife in a little too deep and he hisses, choking down a noise of pain. "You don't have to be quiet."
"I don't want to upset you." He looks down at her and she meets his eyes.
"Just let it out, Bell," she murmurs and he cries out again when the knife digs deep.
The third time, he's twenty and she's sixteen. He chuckles when she brings the knife out and she pauses. "What's so funny?"
"A girl asked," he explains and she freezes. She never thought anyone else would see her marks on him. "She asked how I got such perfect scars."
She trails the knife down his chest, planning, plotting, wanting to mark him in ways that didn't bear asking about, in ways that automatically answered my girl marked me here. She didn't want anyone to ask anyone again. "What did you say?"
He shook his head. "I muttered something."
"What did you mutter?" she said.
"I said love marked me."
When they're on Earth he gets more marks than she leaves on him and it hurts, watching his face and body get scarred, watching him leave his tent, or the dropship, with girls who saw the marks hanging off his arm.
After the longest, most gruelling period of time, she holds out a knife and he swallows hard. "What?"
"Everyone sees your marks, Bell," she said, watching him and offering the knife out again. He takes it cautiously. "Leave marks no one but you will see."
He moves on top of her, pushes up her shirt and presses the littlest kiss to her stomach. It's intimate in ways she knows it shouldn't be, but he can't seem to help himself, and she can't help the little gasp that escapes at his simple touch.
He draws a line up between her breasts. "You're not very creative," she tells the ceiling.
"I don't have to be." The knife turns, digs in and curves. "I just have to make my point."
She holds her breath as the knife meets the line again, then moves away, then digs back in for a second curve.
He's marking his initial into her flesh.
"Please," she whispers. She never says please anymore, too many years of begging to see the outside world and getting no, but here she is, pleading. "Mark me."
He brushes his lips just above the B and oh it's on her breast and oh he's so close to places she wishes he'd be, then he draws away. "Done."
Her chest hurts, her stomach hurts, the mark hurts, but what hurts more is that he's gone, that he stopped as it got good, that he doesn't want what she wants.
"Thank you," she whispers.
He presses a kiss to her and this time it's to her lips and she whimpers and grabs at him desperately but he's already pulling away. "No one sees that, O," he says. "No one but me."
She closes her eyes, exhales a breath. "Just you, Bell," she says, blood seeping into her shirt. "Just you. It's always been just you."