Height
Himiko was, undoubtedly, a tiny girl. At first this fact alarmed Yamato, but after a day of allowing her to play in the park with children her own age he realized that, while amongst the smallest of the group, she was still in the normal, healthy height range for a girl her age, if on the low end of it. He had briefly been alarmed when a woman at the park had seemed surprised to find out that Himiko was six already, having believed her to be younger, but he dismissed it. The fact was, Himiko was just destined to be small.
Undaunted by her diminutive stature, however, Himiko was a clever girl who was more than capable of improvising ways around her height problem. The best shows of her ingenuity could be found in the kitchen: Yamato had allowed her to do certain things on her own, like pour dry cereal or get a cookie. Somehow she had interpreted these small concessions as a sign that she could get these things whenever she wanted without any assistance from her brother.
On more than one occasion Yamato had come into the kitchen to find Himiko climbing on top of chairs, overturned buckets, and the footstool from the living room, pulling herself onto the counter and sitting up on it like it was no big deal when her brother came in. Time and again Yamato had lectured her on the dangers of climbing on unstable surfaces and told her to ask for assistance in the future. It would work for a while, and then he'd catch her doing it again, trying to assert her self-reliance prematurely.
"I can do it by myself!" Himiko would insist, grabbing a box of cereal as Yamato lifted her up off the counter and put her back on the floor, letting her scurry off to another part of the house with her hard-earned prize to be lectured later.
One day Yamato walked into the kitchen to see her grunting and struggling to open the refrigerator door. After observing her for a few minutes, Yamato had gone up and opened the door for her, but she had just pouted and slammed it closed again.
"Aniki, when am I gonna be big?" Himiko whined, standing on her tip-toes for a few moments before nearly falling over.
As she began to slip, Yamato leaned over and scooped her up easily. "Soon enough," he told her. "But if you were bigger, I wouldn't be able to do this!" He put her up on his shoulders and began to walk around the kitchen with her, smiling as she began to laugh joyfully.
When Yamato put her back down on the floor, he found some part of him wishing she'd never have to get any older or any bigger than she was right then.