Freckles
Merry has always felt a little bit protective over Eowyn.
Now I know what you're thinking: don't you have that backwards? Shouldn't it be the tall, stern, noble Shieldmaden of Rohan who feels responsible for this small, jolly, curly-haired hobbit, and not the other way around? And you'd think so, but you'd be surprised. Merry has always had something of an older brother's personality about him—despite not having any siblings of his own—and that, strangely, extended to Eowyn.
It surprised him too, when he first felt it. He couldn't quite place what it was. But something about Dernhelm (because of course he thought it was Dernhelm at the time, and didn't realize the truth yet) gave him a keen urge to stay by this young warrior and look after "him" and make sure no harm came to "him", if his fists and his little sword from the barrow had anything to say about it.
He couldn't quite figure out the source of this feeling until the dark, heavy evening in camp, when Dernhelm stepped behind him while he was eating and tapped him on the shoulder, and Merry turned and saw a fair, freckled hand resting on his cloak.
He nearly jumped out of his skin, thinking an old friend had miraculously returned to him, and after a few stiff words of acknowledgment to Dernhelm he had to turn away and put his face his sleeve to compose himself.
Eowyn's freckles remind him of Pippin.
A/N: It's a personal headcanon of mine that Pippin is covered head to hairy toes in freckles. I have absolutely no evidence to substantiate this belief; it's just the Vibe.