If a white-looking guy like Sgt. Andrew Carter seems an unlikely Sioux Indian, a piece of fan fiction creates a plausible backstory for him:
So Sioux Me
By Atarah Derek
Hogan's men tease Carter and he responds:
LeBeau paused in his dancing and whooping to add, "Or you can show us how to hunt buffalo like you used to do in North Dakota. Hey, you lived in one of those, eh...how do you say, tipi, non?"
Carter rolled his eyes. "I told you guys, the Sioux don't really do rain dances. We do grass dances, fancy dances and medicine dances, but not rain dances. And no, I didn't live in a tipi. Sure, I helped set up a lodge at a powwow one time, but I didn't live in it. Well, not for more than one night, anyway. And if you guys bothered to ask what it was like living so close to the reservation, I'd be glad to tell you, but you're obviously not interested. So just let me finish Schultz's present, for Pete's sake."
Newkirk and LeBeau looked down at their shoes and stuffed their hands in their pockets.
"You know, you're absolutely right, Kinch," Newkirk said. He turned to Carter. "You know we don't mean nothin' by it, don't you, mate?"
"Oui, we're sorry, Carter," LeBeau added.
Carter shrugged. "Aw, it's not a big deal, really. I know you'd never mean to hurt my feelings. And it's really not the worst I've gotten. Not by a longshot."
"White folks back home make fun of you for being part Indian?" Kinch asked.
"No, not really," Carter said. "Most people don't know I'm a quarter Sioux until I tell 'em. Actually, the worst teasing I ever got was when my brother and I would visit our cousin on the reservation. He had some friends who were real bullies."
"Hey, you leave my brother alone!" Andrew snapped, helping Tommy up.
"What are you gonna do, whitey?" the oldest boy challenged. "Tell the Indian grandma you made up?"
"Yeah, you're no Indians," the third member of the gang said with a snarl. "You don't belong on the rez."
Andrew nodded. "I'll never forget I'm part Lakota. And I'll be proud of it always."
Carter smiled. "Heck, I forgave you a long time ago, Louis. I know you guys never really mean anything by it."
"Still a matter of principle, though," Newkirk said. He sat up straight and held up his right hand. "And I swear on me honor that I'll never mock you for bein' Indian again."
"Oui, that goes for me too," LeBeau said.
"Thanks, guys," Carter said.
Below: Carter about to (mis)fire an arrow at a German truck.