Solomon Thomas was the third player taken in the draft, but the 49ers’ rookie defensive lineman might have gone No. 1 if the NFL considered only character.

Just ask his position coach at Stanford, Diron Reynolds, a former NFL assistant. Reynolds has sat in draft meetings in which executives stake their futures on a 21-year-old’s ability to behave.

“There are a lot of people’s jobs and money on the line,” Reynolds said. “And if you want to bet on anybody character-wise, you’re going to bet on Solomon Thomas. That’s an unbelievable investment.”

Or ask the rookie general manager who invested in Thomas after his staff investigated his background.

“There was nothing there,” John Lynch said. “Nothing. ‘What did you find on (Thomas)?’ It was like, ‘He got a B-plus in seventh grade.’”

The gushing prompts a question to the people who knew the model draft prospect when he was in diapers: Was Solomon an easy child?

Their initial response: very loud laughter.

“Solomon,” said his mother, Martha, “was a pain in the ass as a little kid.”

Said his dad, Chris, cackling: “He destroyed everything!”

The oversized child who kicked over other children’s sandcastles and his sister’s Playmobil houses eventually found an outlet for his energy in football, a sport he discovered in fourth grade.

The early returns in the NFL? They suggest Thomas will be the same destructive pain in the butt that he was in college and childhood.

After an impressive preseason debut, Thomas, 21, will play his first game at Levi’s Stadium —15 miles from Stanford’s campus — on Saturday night when the 49ers host the Broncos. The game will come eight days after he had a quarterback pressure that led to an interception on his first pro snap in Kansas City.

“It’s early, but the way he played the other night,” Lynch said, smiling, “I had a bunch of people going, ‘Wow.’ And that’s why he was the No. 3 pick in the draft.”

It’s easy to forget Thomas was the No. 3 selection, given all the attention given to Reuben Foster, the inside linebacker the 49ers grabbed 28 picks later. Unlike Thomas, Foster was a highly scrutinized pick who tumbled in the draft because of concerns about his surgically repaired shoulder and character.

The contrast between their personalities was evident in a joint news conference the day after they were drafted. Foster took reporters through a hilarious, rambling story about hanging up on the Saints during the draft. Thomas’ buttoned-down answers were most notable for the respectful way he addressed reporters who posed questions: “Yes, sir ...”

“I loved the dynamic of those two,” Lynch said. “I said, ‘Hey Reuben, you pull the personality out of him and Solomon, you take care of this kid.’”

Foster has flashed a relaxed confidence this summer — “The ball’s got a thing for me,” he’s said of his interceptions — and Thomas has been admittedly anxious.

Outside linebacker Elvis Dumervil, 33, is among those who have recognized Thomas’ talent — and nerves: “I explained to him, you’re here for a reason. You were the third pick for a reason.”

Before last week’s preseason opener, cornerback Rashard Robinson sensed the rookie’s anxiety and implored Thomas to just have fun.

“I’m feeling more comfortable,” Thomas said. “In the game, I was extremely jittery and nervous. But it’s an NFL game. In my head, I made it out to be something more than it is. It’s like everyone else says: ‘Relax. You’ve been playing football your whole life.’ I’m still in the process of getting that confidence back.”

Thomas can sound vulnerable, not a quality often associated with NFL defensive linemen, but his doubts have not been a match for his drive.

Chris Thomas recalls when his son began playing chess in elementary school and experienced an early setback.

“He said, ‘Dad, I’m not going to lose again,’” Chris said. “And then he went to the finals and won his age division. Solomon just doesn’t like losing, period.”

At Coppell High in suburban Dallas, Thomas’ admission to Stanford was in jeopardy because of a C he had in pre-calculus. He responded by gutting out an “A” on the final after spending four weeks receiving extra tutoring.

“His teacher,” Martha said, “said she’d never seen a kid work so hard to improve.”

Finally, there is the reason he was the No. 3 pick.

Thomas vaulted up draft boards because of his can’t-block-me performance in his final college game in December. He had five quarterback hurries, five tackles and a game-sealing sack in a win over North Carolina in the Sun Bowl.

“When you get a chance to study a team for a whole month, I think you should be able to pick up something, and I think Solomon picked up everything,” Reynolds said. “I think he knew when they were about to sneeze the way he studied the tape. … He has something inside of him that drives him to be the best. He just has this spirit about him that he won’t be denied.”

As a child, Thomas was exposed to a variety of cultures and activities, and those experiences help explain why he notes that football doesn’t define him.

Chris Thomas was an international sales manager for Proctor & Gamble and his job took the family from Chicago, where Solomon was born, to Australia and Stamford, Conn. By the time they moved to Texas when he was in fourth grade, Thomas, who lived in Australia from ages 2 to 7, knew nothing about first downs, but he’d been exposed to Thailand, Tahiti, New Zealand and Spain.

“I think it made him realize that there were just so many different people in the world,” Martha said. “So many different ways to fit in or be. … Neither of our kids have been like, ‘OK, I’ll do that so I fit in.’ He just has never been that kid.”

Thomas is an enthusiastic cook (tarts, French fries and his grandma’s corn dogs are some specialties) who once had a Food Network obsession and a shared dream of opening a restaurant with his sister Ella, 24.

He also has longtime obsession with “Star Wars.” He can do a spot-on Chewbacca roar and dressed up like a Jedi during a Stanford team trip to see “The Force Awakens” in 2015. His love for the genre is rivaled only by his affection for Beyoncé, whose song, “Upgrade U,” was his walk-up music at the draft.

“Other people were like, ‘He’s so big, he’s the No. 3 draft pick and he’s playing Beyoncé?’” Ella said. “I said ‘Yeah, that’s my brother. I love it. That’s so awesome.’”

Thomas said he’s not yet comfortable enough to show his new teammates his full personality. And, as a rookie, he also doesn’t think he’s established a platform to share other issues that are important to him with a large audience. He notes he’s the son of a biracial marriage, and he’s continuing to educate himself on subjects that increasingly have inspired other NFL players to discuss weightier topics than touchdowns.

“We’re more than athletes,” Thomas said. “Just because we’re athletes doesn’t mean we can’t speak. We need to use this platform to change the world. We’re here to be people who can help others and make the world a better place.”

Thomas has clearly grown since he was a 160-pound fourth-grader who was constantly glued to a bench at recess for “playing too rough and stuff.”

In fact, when Dumervil recently was asked for his impressions of Thomas, he began by noting that his young teammate was “obviously raised well.”

Dumervil’s observation got back to Martha Thomas, who knows from experience her son didn’t come by his character naturally.

“My mom appreciated that,” Thomas said. Dumervil “is her favorite 49er.”

Eric Branch is a San Francisco Chronicle staff writer. Email: ebranch@sfchronicle.com Twitter: @Eric_Branch

5 players to watch

QB C.J. Beathard: The rookie is on deck Saturday after he was the No. 3 QB in the preseason opener. He’s expected to play into the third quarter as he competes with Matt Barkley for the backup job.

RB Joe Williams: The fourth-round pick had a quiet training camp, but his speed — his greatest asset — was an eye-opener when he located fifth gear last week at Kansas City.

LB Reuben Foster: The 49ers expect him to play with the mild sprain of his AC joint in his surgically repaired shoulder that he sustained in practice Wednesday. Fans can exhale when the No. 31 pick’s night is done.

QB Brian Hoyer: He had a strong training camp, but there wasn’t a carryover in last week’s six-snap performance. He’ll get more snaps to showcase his stuff Saturday.

CB Rashard Robinson: His up-and-down performance last week included an interception and some reasons for concern. He’ll strive for consistency against Pro Bowl receiver Demaryius Thomas.

— Eric Branch