Marcus Capers was up 10-4 on Klay Thompson in a one-on-one game to 11 when his left calf began to cramp.

It was a muggy summer evening in 2008 after a two-hour open gym, and Capers — then a freshman guard at Washington State — figured he could score one more point for the right to wear No. 1. Not possible. After Thompson capped a 7-0 run by swishing a turnaround corner 3-pointer, Capers, stunned, pulled aside his new teammate.

“No disrespect,” Capers said, “but why are you at Washington State?”

Thompson, with his smooth jumper and NBA genes, seemed tailor-made for such basketball powerhouses as UCLA and Arizona — not an overlooked school in rural southeastern Washington. But the Cougars were the only Pac-10 program to offer him a scholarship, and Pullman (2008 population: 27,245) provided an isolated environment to focus on hushing his doubters.

During halftime of Saturday afternoon’s game against Oregon State at Beasley Coliseum, Thompson, 29, will become the second player in Washington State history to have his jersey retired. Having No. 1 in the rafters will be as much a reflection of what he has accomplished since leaving Pullman as it is of his three years on campus.

As Thompson blossomed into a three-time NBA champion and five-time All-Star with the Warriors, he was a key source of pride for a Cougars program that hasn’t reached the NCAA Tournament since 2008. His quiet approach embodies the principles — hard work, humility, loyalty — that Washington State alums value most.

No. 11 Warriors jerseys dot Cougars home games. Whenever Thompson watches Washington State play Cal in Berkeley, photos of him sitting courtside ripple through the Cougars’ blogosphere.

“The fact that he still represents WSU with pride means so much to everyone that’s ever gone there,” said Daven Harmeling, who was a senior forward for Washington State when Thompson was a freshman. “When he has special moments, it literally makes everyone else’s day better. He probably means more to that school than anybody else in the history of WSU.”

Tony Bennett was on a recruiting trip to Brisbane, Australia, in summer 2007 when he awoke to find a long voice mail from one of his Washington State assistants, Ben Johnson, telling him in excited tones that he had found “the One.”

A shooting guard had decommitted from the Cougars weeks earlier, and Johnson had been asked to find a worthy replacement — a tricky task so late in the recruiting cycle. While watching a top Los Angeles-area AAU program’s B team in an auxiliary gym, Johnson was captivated by the gangly teen whose shooting motion didn’t change, his shoulders relaxed and follow-through minimal regardless of the situation.

Little more than a month later, having been rerouted after he missed his connecting flight from Orange County, Thompson arrived in Pullman on a chilly September morning in socks and flip-flops. Over the next 48 hours, he seldom said a word as coaches and players took him bowling, showed him the local Denny’s and pitched him on becoming the face of a Pac-10 program.

At one point, Johnson half-joked to players that, “If anyone can make him smile or laugh, they get $20.” No one would collect that money. After sinking more than 20 consecutive 3-pointers in his flip-flops at the 11,671-seat Beasley Coliseum, Thompson went to forward Charlie Enquist’s house, where he spent hours trying to beat the Outlaws’ “Green Grass and High Tides” on the video game “Rock Band” while his hosts watched in bewilderment.

“He was really good, but he couldn’t beat it on expert, and he was getting frustrated,” said Abe Lodwick, a former Washington State forward. “I remember thinking, ‘Man, this kid is competitive.’”

After driving Thompson to the airport, Johnson told Bennett, “There’s no way he’s coming here.” A few days later, Thompson verbally committed to the Cougars over offers from Notre Dame and Michigan. Coaches had sold him on the idea of showing UCLA and USC what they missed out on by not recruiting him.

The scenic beauty of Pullman, a town surrounded by wheat fields with a population barely bigger than the university student body it houses, resonated with Thompson. Though he had hardly spoken on his official visit, he noticed how Bennett’s program — fresh off its first NCAA Tournament appearance in 13 years — had galvanized a fan base.

“They embraced the Cougs like none other because that’s all that’s really out there in eastern Washington, Cougar Pride and the Palouse,” Thompson said. “That’s what drew me. Especially coming from Southern California, it’s nice to get away from the grid and the city.”

For much of his first year at Washington State, Thompson spoke only when asked a question. But as the months passed and he got more comfortable, he began to reveal his goofier side.

When the campus emptied for winter break, Thompson passed the doldrums by starting snowball fights or exploding clementines with fireworks, a concoction he and his teammates called “clemen-ades.” If not studying or putting up shots in the Cougars’ practice gym, he was probably challenging friends to video-game tournaments.

When Thompson was beginning to receive national attention his junior season, he attended a house party with walk-on guard Ben Loewen and was greeted by cheers. After a minute or two, Thompson turned to Loewen and said, “Let’s go play ‘Mario Kart.’”

About a month later, when Thompson got bored with “Mario Kart,” “Super Smash Bros” and “Call of Duty,” he bought a bow and arrow and converted the living room of the duplex he shared with three teammates into a makeshift archery range. A bed was propped up behind a cardboard box with a drawn-on target to prevent holes in the wall.

“You had to make your own fun,” Loewen said. “But honestly, that was part of the fun of it and part of why I think Klay liked it so much.”

Less than two weeks after Thompson’s freshman season ended with a loss to St. Mary’s in the first round of the NIT, Bennett accepted the coaching job at Virginia. Leaving Thompson, who already looked destined for a long NBA career, made his decision tougher.

After agreeing to replace Bennett in Pullman, new head coach Ken Bone left Thompson six voice mails in a four-week span. Worried that his best player would transfer before he even talked to him, Bone called Klay’s father, Mychal, who told him Klay was bad at returning calls and was staying at Washington State.

After getting cited for marijuana possession late in his junior year, Thompson was suspended for the Cougars’ regular-season finale against UCLA — a devastating blow for a team on the NCAA Tournament bubble. Minutes before tip-off, Thompson asked the PA announcer for the microphone and, while fighting back tears, apologized to the Beasley Coliseum crowd.

“Here is this shy guy going in front of thousands of people to make sure he owns the moment and takes responsibility for what happened,” Lodwick said. “Klay gained the loyalty of a lot of people that day. He really just showed how much he cared about Washington State, which I think has proven itself over and over again.”

Five days after the Cougars fell to the Bruins in overtime, Thompson returned to score 43 points in a conference tournament quarterfinal loss to Washington. Even though he was widely projected as a lottery pick, Thompson loathed the thought of letting down Washington State again. In the weeks before he finally declared for the draft, when teammates would ask what he planned to do, he said he wanted to come back for his senior year.

The NBA had been Thompson’s goal since he was a young kid idolizing Kobe Bryant, but he didn’t feel quite ready to leave his simple life in Pullman. A place largely ignored by the rest of the world had allowed him to be his quirky self and build lasting friendships.

Thompson gets Harmeling — a high school coach in Vancouver, Wash. — tickets whenever the Warriors play in Portland. Loewen and Lodwick know that, whenever they three-putt in a round of golf, Thompson needs to hear about it. When Capers — a shooting guard in the National Basketball League of Canada — visited Thompson in Toronto for Games 1 and 2 of the NBA Finals last spring, they played “Super Smash Bros” in the team hotel for hours.

“If I could go back and do it again, I would choose Pullman again every day of the week,” said Thompson, whose Washington State teams never appeared in the AP Top 25. “It fit my personality perfectly, and I was able to make relationships that I still have to this day.”

Connor Letourneau is a San Francisco Chronicle staff writer. Email: cletourneau@sfchronicle.com Twitter: @Con_Chron