Kyle Boyd still can’t say exactly why he did it.

Some days, the decision still puzzles him. He knows he was in a position of privilege — not many high school football players are offered scholarships at Power Five programs — but also, that something felt off. And so six years ago, despite a signed National Letter of Intent that signaled his commitment to play football at Baylor, Boyd declared he would walk away from the game and join the Marine Corps.

His parents spent the next few months desperately trying to change his mind. The announcement “came out of left field,” said his mom, Lorena. She always thought the American flag that hung in Kyle’s room, the one with a Marine Corps logo, was just his way of being patriotic.

In hindsight, maybe it shouldn’t have caught Doug and Lorena Boyd off guard. Kyle didn’t grow up in a military family, per se, but Doug is in his 27th year as a patrol officer for the Mesquite police department, “serving and protecting, ready to sacrifice his life at any minute,” Kyle said, and that’s left an impression.

Kyle, a 6-foot-2, 230-pound linebacker from Mesquite Horn High in Mesquite, Texas, explained it like this: “I’m a man now, I can make my own decisions,” he told his parents, “and I feel called to serve my country.”

But he never imagined it would have turned out like this. After four years in the Marines, the former three-star prospect is now the starting fullback for the Baylor Bears, a 24-year-old sophomore walk-on with one of the best stories in college football.

He’s also a popular target for a variety of geriatric-themed jokes.

“I’m teased in the locker room all the time,” Boyd said. “The other day someone told me for Halloween all I need is a cane, and I can go as an old man.”

In reality, Boyd’s experience and maturity are exactly why he’s a perfect fit at Baylor.

“I saw his discipline and toughness, that he knows how to grind … and thought he’d be a great fullback,” said Bears coach Matt Rhule, himself a former walk-on linebacker at Penn State. “Whatever we ask him to do, he doesn’t complain.”

That makes sense though, Rhule said, when you consider Boyd’s alternative: “I mean, we ask our kids to do a lot, but we don’t shoot at them,” Rhule joked.

It’s true that after four years of grueling military drills, football is mostly a cakewalk by comparison. Whenever Boyd starts to hurt or get tired, he thinks to when he was stationed in the mountains of South Korea. Temperatures dropped to single digits at night, and yet the Marines would often strip to their shorts and practice cold river crossings, sprinting through the snow and icy water while their bodies went numb.

In the middle of fall camp, when the temperature creeps into the triple digits and coaches scream about running plays correctly, walk-ons often wonder what they got themselves into. Boyd had that moment during boot camp.

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“When I told people I was going to join, everyone kept telling me I was crazy to give up a football scholarship to go get treated like dirt 24/7,” Boyd said. “In boot camp, I’d be laying in bed at night thinking, ‘Wow, how did I get here? It’s 3 a.m. I haven’t really slept in a few days, and a drill instructor might come in at anytime and make me do push-ups.’ ”

But he kept going, his spirits buoyed by letters from his parents, who wrote to him daily. After three months of boot camp and two months of infantry training in San Diego, Boyd shipped out to the Hawaiian island of Oahu, where he visited the grave of his great-grandfather, a Marine killed in action during the Battle of Iwo Jima. Over four years of service he deployed twice, first to Australia and New Zealand, and then to Japan and South Korea, training other militaries in American tactics. He never saw combat.

Two years into his four-year military contract, Boyd got the football itch again.

“I just kept thinking, I love this game, I miss it and I want to give it a shot,” he said. “I told my buddies, I’m gonna go try to play because if I don’t, I’ll regret it for the rest of my life.”

His fellow Marines laughed and rolled their eyes. In the military, Boyd said, it’s common for soldiers to regale each other with stories of high school sports glory, telling their bunkmates some version of, “Oh I was all-state in football and wrestling and track. Every college wanted me.” But with Boyd, it was true.

Coming out of high school, Boyd had a handful of Division I offers, including Baylor, Iowa State and Wyoming. When he decided he wanted to pursue football again, he spent his downtime emailing coaches, trying to convince them he was worth a spot even though his “five-year old film absolutely stunk,” Boyd said. Fortunately, his body composition hadn’t changed much. Though he got down to 195 pounds during boot camp, Boyd maintained a weight of 220 through his service.

Baylor, riding high off Big 12 titles and BCS bowl appearances, told Boyd he was still welcome in Waco, but he’d have to come as a walk-on. Because he’d get financial aid through the GI Bill, Boyd didn’t have a problem with the tweaked game plan. On Sept. 26, 2015, with almost two months of leave built up, Boyd attended his first game in McLane Stadium, a 70-17 blowout win over Rice. He stood on the sideline with other Baylor recruits — him, a 22-year-old almost-military-veteran with a full beard, next to a bunch of scrawny 17-year-olds.

By October, Boyd had been granted early release, honorably discharged from active duty from the LIMA Company as an E-4 Corporal (Boyd is now midway through his inactive reserve duty, another four-year term). He took a few credits at a local junior college spring semester, enrolling as a full-time student and football player in July 2016.

Despite former coach Art Briles’ firing in the wake of a massive sexual assault scandal that rocked college football, Baylor maintained its commitment to Boyd. In fact, after a mass exodus of players last spring following Briles’ termination, the Bears boasted more than 30 walk-ons under interim coach Jim Grobe during the 2016 season. Boyd's also enjoyed the transition to fullback this season, proud to take over a position “forgotten by most of football, where you have to be gritty and nasty, willing to put your nose in somebody else’s pads.”

He understands that for many, Baylor has become a dirty word in the college football lexicon. But he takes particular pride in helping rehab the Bears’ image, and is grateful for the opportunity to be a part of another brotherhood.

His favorite moment as a football player involved a collision of those two brotherhoods, in fact.

In Baylor’s second game of the season against SMU last year, Boyd was asked to carry the American flag on to the field in honor of Military Appreciation Day. The moment overwhelmed him. “Just thinking about, the hair stands up on my arms,” he said. “It’s an incredible feeling of adrenaline.”

His parents remember it well, too, and his mom’s voice catches when talking about it. She meant to take a million photos and a video to commemorate, but she couldn’t because in the stands, she was shaking and crying too hard. When Kyle first told his parents that he planned to volunteer for one of the most dangerous professions, Lorena begged him not to go, terrified that the worst could happen. She didn’t understand why he’d give up college football, or why he’d be willing to make the ultimate sacrifice.

But on Sept. 10, 2016, she knew Kyle’s journey worked out exactly as it was supposed to.

Know of a good walk-on story in college sports? Lindsay Schnell wants to hear it. Email her at lschnell@usatoday.com