(A/N: Wybie is here, so it's basically Movie-verse, although I took some things, like her day-glo green gloves, from the book)
Coraline had always thought the slouch came from the old skull mask, that he would outgrow it someday. She had tried the mask on once, the two of them sitting on Wybie's bike, waiting for her parents to pick her up after school. The cold metal had pushed against her ears and pressed the dragonfly clip into her temple. Her eyes boggled at the blurring, distorting lenses, and she clumsily found the crank. Every turn became clearer, every turn brought the image of Wybie's slumped shoulders and crooked smile closer. She laughed, and he followed, pulling the mask upwards. He paused when it was halfway up, when Coraline was still blinded by the hard metal shell, and kissed her. She froze, and slowly moved her hands upwards, her fingers (dressed in day-glo green gloves) curling over his (clothed in skeleton leather) over the mask. She pulled the mask completely over her head, pulling his arms and so him, upwards and towards her. She could smell his breath. It smelled like the tater tots in the cafeteria, and nervousness. They kissed again.
The point being, of course, that the mask was heavy, and could easily explain his bent form. But then again, he never did straighten out, even when the teacher called on him in class to explain what-exactly-you-are-doing-with-that-slug-in-my-class-Mr.-Wybourne. He didn't lower his shoulders and straighten his back, even when he needed something on a higher shelf, even when he was teased, even when Coraline's limbs stretched with adolescence and she rocketed well above him. Even when Coraline replaced her dragonfly with a bobby pin and outgrew her day-glo gloves, removing the paint from her nails. Even when Wybie stopped following her around in the fog and had growth spurts of his own, learning to keep a bit quieter in class. He never did get rid of his slouch, nor Coraline her beetle blue hair. Even though it meant he was easily influenced to do her math homework when she gave him back massages. Even though her hair dye was painting her scalp a light cobalt.
Coraline did notice that his slouch, at least now that he has a few more inches on him, made his lips align perfectly with hers.