Byleth slathers some of the lotion onto his skin, rubbing it in to protect himself from the sun. There is, of course, a portion of his back that he can't easily reach without doing some odd and uncomfortable contortions, so it's reasonable to ask his companion to get those places for him. Dedue wades over to where Byleth stands, and takes the little bottle to squeeze some out onto his fingers.

He dabs the cream onto sections of Byleth's back and a little at the nape of his neck, then rubs it in until it disappears into his skin. It's mostly the middle of his back that he wouldn't be able to easily reach, so Dedue focuses on evenly covering that area. The faint scent of pineapple and coconut hangs in the air around Byleth, sort of making him smell like a fake pina colada.

"There." He turns the bottle around so that he can read the directions printed on it. "You will need to reapply it later, as it will be washed off as you swim."