BOSTON -- When Carsen Edwards was 4 years old, his parents -- Carla and James Edwards -- signed him up for full-contact football.

At the time, they didn’t know their son would become a professional athlete. They didn’t know he’d lead the Boston Celtics’ Summer League squad in scoring, that some fellow NBA rookies would bet on him to have the best career in a rookie class that includes Zion Williamson. They didn’t know playing football as a high-school sophomore would indirectly light an angry fire in Carsen’s belly -- a flame that helped power his rise from a promising high-school player to a March Madness sensation.

When they dressed Carsen in a football uniform for the first time, Carla and James weren’t really thinking about his athletic future. More than anything, they wanted to make sure Carsen survived life with his older brother Jai.

“We just wanted him to learn to do his football thing," Carla said, “or he was going to get killed here at home.”

Carsen (left) and Jai (right) Edwards.

At 6-feet tall and 200 pounds, Edwards is built like a running back, a comparison he hears often. He uses that bulk to his advantage -- he absorbs contact on both ends well, and he’s powerful enough to create space despite his lack of height (fellow Celtics rookie Romeo Langford noted with a smile that Edwards “definitely isn’t fake strong”).

While Edwards might seem naturally tough, he developed the attribute during his formative years. Jai loved being physical -- he sought out contact, wrestling first with his older sister Madison, then later with his younger brother.

Carsen didn’t like wrestling or playing with Jai’s wrestling toys. But more than anything, Carsen wanted to be like Jai, even if it meant bruises and cuts and overmatched wrestling bouts.

“He would sit next to me and play next to me until I was done, then he’d hang his stuff up,” Jai said. “He always tells me that now that we’re older. Those type of things that he didn’t like, he did because he wanted to hang out with me.”

The brothers became inseparable, and when Carsen turned 4, he joined Jai’s football team. The gap in development between 4- and 6-year-olds is cavernous, and sure enough, in Carsen’s first game, a bigger boy tackled him hard.

Jai helped Carsen off the field.

“That’s okay, Carsen,” Carla remembered Jai telling the weeping 4-year-old. “That’s okay. I’m gonna get him.”

Playing with (and against) older kids, Carsen developed his own love for sports, and as the brothers grew, they worked on their games around the house. They set up couch cushions as yard markers in an empty room and fought for first downs. Carla worried about Carsen’s tackling, so she and James ran tackling drills for him with Jai in the backyard.

At first, Jai let Carsen bring him down him without much resistance.

“But my parents would be like, ‘Jai, run over him,’” Carsen said. “We was out there all night until I was able to tackle him.”

When the boys started basketball, Carla told James that she didn’t think Carsen rebounded enough, so James took the boys out to the garage and threw a basketball up on the roof. Carsen and Jai waited blindly for the ball to roll off, then raced to it and wrestled for possession.

As Carsen got better at basketball, Jai helped him work out at a nearby Lifetime Fitness. When Jai grew bored with drills, he challenged Carsen to play 1-on-1.

The ensuing games were trench warfare. Carsen could score on Jai, but Jai was stronger and he tried to outmuscle his younger brother, who in turn felt the need to hold his own. Inevitably, when Carla arrived to pick the boys up from the gym, one would be limping while the other sported a busted lip or fresh scratches.

Both Carla and James grew accustomed to hearing about their warring sons from concerned onlookers, and eventually they told the boys they were no longer allowed to play against each other (a rule that still stands when Carsen goes home). If the boys were going to hoop together, they had to be on the same team.

“They would come home all scratched up, and then they are the best of friends,” Carla said. “Carsen thinks the world of Jai. It doesn’t matter what Jai does, they just have that really good, close relationship.”

While the games themselves were physical altercations, the brothers never actually came to blows.

“We wouldn’t fight," Carsen said. "We’d just play 1-on-1.”

Forged in the fires of his parents’ living room, Carsen was an excellent athlete by the time he started at Atascocita High School, just outside Houston. He made the varsity basketball team as a freshman, and that summer, he decided he wanted to play football to spend a final season with Jai, then a senior (Jai is now a senior defensive back at Tarleton State -- a Division II university in Texas). Despite not playing football as a freshman, Carsen made the varsity team.

Football season bled into basketball however, and Atascocita boys basketball coach David Martinez made a decision to send his sophomore guard down to junior varsity for a few games to get his legs back under him.

For Carsen, that decision lit the match.

Carsen and Jai Edwards.

Asked whether Edwards took his brief demotion to JV personally, Carla chuckled for 15 seconds, repeating, “Yeah. Um, yeah. Yeah,” as she searched for the right words.

“He did take it pretty personal,” she said finally.

Martinez didn’t mean for anything to be taken personally. Edwards spent roughly 10 days with the JV squad before he moved back up to varsity, where he spent the rest of the season. But to this day, Martinez doesn’t feel like he can joke about his decision with Edwards (“He don’t like that conversation”).

“I was like, ‘I’m just trying to get you some legs, man,’” Martinez said. “But as a 15-year-old kid who is talented, he doesn’t see it that way.”

At first, Edwards was so discouraged, he wanted to transfer schools, or even quit basketball entirely. James and Carla wouldn’t let him.

“We told him, ‘You’re not going to take the easy way out,’” Carla said. “’If you quit basketball, you’ll have a pattern of quitting. You’ll want to quit your job whenever it doesn’t work out. If your relationship doesn’t work out, you’ll want to quit that. Try to work it out.”

Edwards stuck with it. By the time the playoffs rolled around, he had forced his way into the Eagles’ starting lineup. His role on the team as a future star was clear, and Martinez marveled privately to his assistants at the changes they saw.

But Edwards wasn’t satisfied. The demotion woke something in the 15-year-old, and every morning for the rest of his sophomore year -- and then for the rest of his high-school career -- Edwards could be found at the gym doors at 6 a.m., waiting for a coach to let him in. He quit football, and he didn’t attend a game his junior and senior years. While his friends experienced Texas’ legendary Friday night lights, Edwards lifted weights or got shots up alone.

Now Edwards believes being demoted made him better.

“Not because, ‘Oh, it made me want to prepare better,’ nah," Edwards said. "‘You did this, now everybody that’s on varsity, I’m going to kill them, regardless. I don’t care. Even my brother, I’m gonna kill him, too. I’m gonna kill everyone out here because you think they’re better than me.’

“I get worked up thinking about it now. My thing was just to make him feel so stupid for not letting me play.”

Martinez knows how the demotion was received.

“Looking back, would I have done it differently? I don’t know,” he said. “But my intention was to get two legs under him, let him get some shots off, let him play. So that was kind of a turning point because I think it really, really motivated him to be like, ‘I’m going to show you. I’m going to show you.’ It kind of just worked out from there.”

The relationship seemed to warm a bit Edwards’ junior year. Carla and James reminded their son that while he disagreed with Martinez’ decision, the coach was simply doing his job and that Edwards shouldn’t disrespect an adult.

Meanwhile, Edwards’ bitter hours in the gym paid off exponentially. After averaging five points per game as a sophomore, he put up 23.6 points and 4.9 assists per game as a junior, and 26.3 points per game as a senior. While Edwards was never tall, he continued to hit the weight room aggressively, and he could bulldoze skinnier opponents around the rim while showing off the deep 3-point range that wowed March Madness viewers three years later.

Martinez made a rule for Edwards: Deep 3-pointers were fine, but if he missed two in a row, his next shot had to be at the rim.

“He understood that, and he did that, and it really worked out for us,” Martinez said.

Edwards’ profile grew, and Atascocita became a powerhouse. During Edwards’ senior year, the Eagles were the No. 1 team in the country -- undefeated and rolling through national showcases. After one tournament in Missouri, fans swarmed Edwards, and he signed autographs for everyone who asked. When the team’s ride back to the hotel arrived, Martinez went to locate his players.

“I turn back and I look to try to gather guys, and (Carsen) is picking up trash,” Martinez said. “He’s helping the custodian pick up trash, and he has teammates doing it, too. I thought that showed a lot about him, the type of character he has.”

The Eagles powered their way to the state finals and had a chance to clinch an undefeated season in a game Edwards later described to the Purdue sports website as the “biggest sports thrill I participated in.”

In the championship game, however, everything fell apart: A key contributor was sick, and both Edwards and one of the team’s big men got in early foul trouble against future Duke center Marques Bolden. Atascocita fell 73-54, a disappointing end to Edwards’ high-school campaign.

Martinez made his way around a subdued locker room, and four years later, he recalls exactly what Edwards told him.

“He won’t even remember this, but I remember it: ‘Coach, thanks for letting me play my game,’” Martinez said. “That’s exactly what I was trying to do. I wasn’t trying to handcuff the kid. I knew how good he was.”

Carsen Edwards' family: Madison, James, Carla and Aspen Edwards.

In June, Edwards watched the draft with a small group of family and friends at a hotel in Houston. He slipped through the first round despite plenty of mock-draft projections in the late 20s, before the Celtics scooped him off the board with the 33rd pick -- acquired from the Philadelphia 76ers in a bit of smart maneuvering that involved Matisse Thybulle.

Shortly after the draft, Edwards was invited to throw out the first pitch at Fenway along with fellow Celtics rookie Grant Williams. Jai accompanied him, and the experience was surreal for both brothers.

“Of course, he didn’t want to say it when we was there, but when we got up to the suite, he was like, ‘Bro, it’s crazy that I’m actually here doing this,’” Jai said. “... It’s crazy because when he got drafted, as the weeks went on, we were still like, ‘Hey man, it’s unbelievable that he got drafted.’ He would call me and say, ‘Bro, it’s crazy that I’m on the Celtics.’ He still does that today. He’ll call and be like, ‘Bro, it’s crazy that I’m here.’ It’s unbelievable still.”

Carla and James refer to the Celtics as Carsen’s “work” when they talk about Summer League. As always, they expect Carsen to be a professional, and they raised him not to quit his job.

Not that anyone expects Carsen to quit. The first day he arrived at the Celtics’ training facility, Edwards sent his parents a picture of his locker, which had his name and number over the top.

“He mentioned he really teared up," James said. "I teared up just seeing it there, just to see a locker. He sent it to his mom, and she teared up. It was pretty amazing.”

When Edwards arrived at Purdue, a concerned assistant coach once called Carla and James asking how he could convince his freshman guard to dial back the physicality, that he didn’t need to prove himself so aggressively against teammates in practice (“Well, we don’t know what to tell you,” Carla remembers telling the coach. “When you separate the teams, it’s just called competition for us.”)

Now that he’s in the NBA, Edwards feels less like he has everything to prove and more like he has a lot to learn.

“The only way you can learn is from guys who have been through it,” Edwards said. “So all these people who have been here that we’re playing with, you want to compete, but you also want to learn and hear them out and listen to them. That’s why I feel like it’s different. These guys, especially on this team, there are so many good guys who have been through it, been All-Stars, been in the playoffs, stuff like that, so just being around those guys and still trying to prove yourself and earn stuff, but also take a step back and learn.”

His family expects the fire to continue burning.

“We always talked about stuff we wanted to accomplish, and he’s accomplishing it," Jai said. "It’s amazing. I hope he keeps going and keeps accomplishing things. I know he will. He won’t stop.”