When he was 13 years old, dealing with the harsh realities of his family’s life in Oakland, Damian Lillard made a promise to his mother. He said he would go to college, graduate, move on to the NBA and become Rookie of the Year.

It was quite a statement from a kid barely getting recognition as a player, but it stemmed not from arrogance. It was a matter of pride. Oakland pride. As the Warriors-Portland playoff series opens Sunday at Oracle Arena, fans will discover, once again, exactly what that means.

The first time Lillard played at Oracle, so close to the Brookfield neighborhood of his youth, he scored 37 points. He’s been torching the Warriors ever since, including a 51-point masterpiece in Portland on Feb. 19, one of Golden State’s nine losses in a 73-win regular season. Coach Steve Kerr’s team just finished dispatching a bickering, dismantled Houston team in the first round of the playoffs, but the mood is about to change.

Lillard will make certain of that. The Warriors might prevail — Stephen Curry’s health will be an overriding factor — but not without Lillard leaving his heart and soul on the court.

Oakland pride

Everything about Lillard’s presence speaks to his upbringing. He wears the number 0, representing the letter O. In the ornate design of tattoos across his chest, most prominent is “Oakland” and “Heart of the City.” The words lie atop the tangled branches of a tree, its roots plunged into a heart-shaped base.

This is a serious man. Lillard fulfilled all of those promises to his mom, and he now joins Curry, Russell Westbrook and Chris Paul among the NBA’s point-guard elite.

With that reputation comes a ton of responsibility. It’s not just about scoring, but engaging teammates in the purest possible flow. Lillard can call up a 30-point performance virtually at will, but when the situation dictates, his selfless nature takes over. That was glowingly evident in Portland on Friday night, when the Trail Blazers eliminated the Clippers in a tense Game 6. Teammates (notably CJ McCollum) took most of the big shots down the stretch, and Lillard was a most willing facilitator.

“I was really proud of what our team did tonight, the way we executed down the stretch,” Lillard said afterward. “We didn’t panic when they put up a fight for us. We did the things necessary to win the game.”

Facing the Warriors in Oakland, he said, marks “an opportunity to play against the best team in the league. That’s the thing I’m most excited about. Secondly, to be able to play in front of family and friends. Since college, they haven’t been able to see me play a lot. To be able to come home and play on that stage, there’s no greater feeling.”

At the Warriors’ practice Saturday, Kerr said Lillard “has range out to 26, 27 feet, but he’s also a bulldog. He’s so strong, he can get right to the front of the rim. It’s a pretty rare combination to have that kind of speed and power and shooting range.”

Line of point guards

Oakland is rich in point-guard tradition: Jason Kidd, Gary Payton, Lester Conner and Brian Shaw all grew up there, as well as some hard-luck cases who fell by the wayside, such as Demetrius “Hook” Mitchell, a high-flying dunk artist who stood just 5-foot-9. To be tough, fearless and confident — that’s the common thread.

“Damian plays like the rest of us did, with that little swagger and that complete calm,” Payton told SI.com. “He doesn’t get rattled. That quality comes from Oakland, from the neighborhoods, from going into other gyms and getting challenged.”

Lillard has spoken of an unsettling episode from his teenage years, when he was robbed at a bus stop by three men, one of whom pulled a gun. In an interview with ESPN’s Doris Burke, Lillard said growing up in Oakland “just made me tough. You see a lot of things, you’re around a lot of things. You’ve got to be able to handle it. Everybody’s going through it, so you can’t feel sorry for yourself. If something bad happens, you can’t be the guy that’s sticking out, uncomfortable. You’re raised there. It prepares you to survive anywhere. You can handle any moment.”

Perhaps only Lillard envisioned his magnificent future. He learned to shoot on a milk crate rigged to a telephone pole at his grandmother’s house. He transferred from St. Joseph to Oakland High to gain more playing time and was an all-league choice his senior season, but the prospect-scouting outlets were only mildly impressed. On the local AAU circuit, invariably dominated by the Oakland Soldiers (LeBron James, Chauncey Billups and Brandon Jennings are just a few alumni), Lillard played for the Rebels, housed in ramshackle facilities and rarely drawing much attention.

Promise kept

Among the scant few colleges showing interest, St. Mary’s among them, Lillard chose Weber State, strictly out of loyalty to coach Randy Rahe, the first to recruit him and the one most attuned to his character. Rahe made him a starter immediately (he might have had to sit a couple of years at a Pac-12 school), and by his junior season he was averaging 24.5 points a game and shooting 46.7 percent from the floor. Sacramento had the No. 5 pick in the 2012 draft and misfired, taking Kansas forward Thomas Robinson, allowing Portland to grab Lillard at No. 6, just ahead of the Warriors’ selection of Harrison Barnes.

Lillard was about to become a full-fledged star, but he never forgot that promise to his mother. He had three classes to finish to earn a professional sales degree from the College of Applied Science and Technology at Weber State, and he spent much of the 2014 summer on campus. With the NBA season at hand that fall, he continued his studies remotely, and in May 2015, two days after Portland was eliminated from the first round of the playoffs by Memphis, he received his degree in a cap-and-gown ceremony.

Lillard’s parents separated when he was young, but they remained united in looking out for him. One night during a big game in high school, Lillard hit the decisive shot and noticed, through the crowd’s frenzy, that his father had no reaction beyond a nod of the head. The message, one he never forgot: Act like you’ve been there before.

‘Biggest shot of my life’

Fast forward now to Game 6 of the 2014 first-round series against Houston and the signature moment of Lillard’s career. With 0.9 of a second left, the Rockets holding a two-point lead and the ball coming inbounds for a halfcourt set, Lillard stood calmly alongside his defender, Chandler Parsons, looking completely disinterested. Suddenly, he bolted to the center of the court, frantically clapping his hands, demanding the ball. With enough time only to catch and shoot, he hit the 27-footer that won the series and sent Portland’s Moda Center into an uproar.

As everyone in the place went wild, especially Lillard’s teammates, he stopped cold and let the celebration come to him. Only the look on his face, one of fierce defiance, revealed what raged inside him.

“Definitely the biggest shot of my life,” he said afterward, and then a smile. “So far.”

And now comes the most important series of Lillard’s life, with the intriguing potential of a backcourt clinic. Get well, Stephen Curry.

Bruce Jenkins is a San Francisco Chronicle columnist. Email: bjenkins@sfchronicle.com Twitter: @Bruce_Jenkins1