His mother's cry of "Sa-yu!" defines his childhood.
She's heedless, his little sister, and doesn't think at all. Light is the smart one, and thinking is his job. Sayu gets into trouble, stumbles into the same traps again and again, needs reminding of the smallest things - or at least, is reminded of them, just in case. Light, on the other hand, sees all the angles, plans ahead, and keeps her out of trouble when he can. It works well enough, considering that he's only nine, and she's only just turned six.
"Honestly, Sayu, you know better than
to pick things up from the floor." "Yes, yes, yes.
But look, look what I found, oniichan?"
This time it's a tennis ball she's found in the park, nestled in the crook of a tree. It's almost new, lost, discarded; someone's batted it out of sight forever. She likes the bright yellow baseball fuzz, but he likes the way it bounces, and the answer to take home to his mother is obvious: someone let them keep it. Sachiko will blink twice, but he knows she won't take the ball from him - at some point, it's ceased to be Sayu's ball. So there it is, bouncing gently back and forth - right into the central point he's marked in the box, right back out to his hand. His mother looks over from the sofa and her tea and her novel, and offers a quiet admonishment. "Light, don't bounce your ball in the house."
Disappearing it into his pocket, he apologises, well-trained. "Sorry, kaasan." She'll never have to tell him again.