Samantha
The first time Dean caught Sam putting on stolen black nail polish he raised his eyebrows, smirked his usual smirk and told him to make sure he got the whole nail. Sam spluttered, dropping the tiny brush onto their shared motel bed and yelled at him to get lost, that he only wanted to see what it was like, that it was only curiosity that made him try it. Dean's smile softened, and he told Sam, in all honesty, that he didn't give a shit if Sam wanted to wear nail polish or take a class of freaking Zumba, and that he should do whatever made him happy. Sam looked at him with distrusting eyes, and then his expression brightened and he went back to painting his toenails.
The second time Dean saw Sam coloring his finger nails, it was with a small bottle of deep purple nail polish Dean had stolen from a gas station convenience store two states back. As usual, their Dad was out on a hunt and the two brothers huddled close on the couch, Sam therapeutically humming to himself as he put on a second layer and Dean passing a hand through Sam's shaggy mop of brown hair.
"Missed a spot," Dean commented as he peered over Sam's shoulder at the work he'd done on his nails. Sam glared at him, but brushed the tip of the brush over the gap anyway.
The third time Dean stumbled in on Sam doing someone that was perceived as girly by the rest of society, he just shrugged, unsurprised, and helped Sam hold the small mirror while he used a pair of tweezers from their medical kit to pluck his eyebrows into two perfectly-shaped, rounded lines.
They never told their Dad about it. He would have a stroke if he ever heard his youngest son sometimes liked to dress like a girl. They didn't have money for a decent wig, so Sam let his hair grow out a little. Dean still stole nearly-perfectly-fitted dresses for him from the woman's department at the local mall, taping tin foil to the inner side of his backpack so it wouldn't beep at the entrance and wrapping them in old newspaper like they do with presents at Christmas. Sam still liked to be called Samantha sometimes when their Dad was away and he would paint his nails, put on lipstick and the pair of heels Dean's girlfriend at the last town they visited will probably never miss – she had so many of them, it would be a wonder if she ever noticed they were absent.
"Here's a buck. Go get a snack for you and Sam while I pump up," their Dad said once, passing on a dollar bill to Dean.
Sam was instantly out of his seatbelt, reaching for the door. They've been on the road non-stop for over five hours. Dean knew his brother, and he knew that if he had to stay in the car any longer they'd both be in for some serious bitching.
"Come on, Samantha. Let's go get you something pretty," Dean turned his head to look at Sam, smirking.
Sam half-glared, half-smiled at his brother. Sam had told him once that it was nice that Dean felt comfortable enough with the whole thing to make jokes about it. Dean had replied that he wouldn't be Sam if he didn't like to drag up every once in a while.
The truth was, Dean just liked seeing Sam being himself, being honest and happy and unafraid. And if he had to dress up like a girl sometimes for that to happen, well, Dean would just have to stock up on acetone and eyeliner on every gas station they goddamn pass.