The new Mr. October? Bryce Harper breaks through to the biggest stage at the peak of his powers.

HOUSTON — As the ball took flight on its path to the left-field bleachers, Nick Castellanos didn’t take his eyes off the man who hit it.

Castellanos has known Bryce Harper for 15 years, literally half their lives. He has marveled at his talent on a baseball field. Big swings? There have been plenty. Clutch hits? There’s a long list. Tape-measure home runs? No doubt.

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But Castellanos never saw Harper react like this to any of his previous exploits. After banging the biggest homer of his life — a two-run shot that pushed the Phillies to the verge of winning the pennant in the eighth inning Sunday at Citizens Bank Park — he stood and watched it for a beat, then looked into the dugout, put his head down, and circled the bases. There was no bat flip, no yelling, no excess celebration.

“The way that he was able to immerse himself in the moment and stay focused and calm was [bleeping] incredible. Please use those exact words,” said Castellanos, the Phillies right fielder. “He was the calmest person in the stadium. And I think that’s a lot of growth on his part.”

As he nears the end of his 11th major league season, Harper is a fully formed superstar, which isn’t to say that stardom is anything new for him. He was a teenage phenom in high school, the first overall pick in the 2010 draft, a major leaguer at age 19, National League Rookie of the Year in 2012, league MVP in 2015, the recipient of a $330 million contract in 2019, an MVP again last year, and a seven-time All-Star before he turned 30.

But in this month, at this moment, on the eve of the 118th World Series, Harper is at the absolute peak of his powers. He’s 18-for-43 (.419) with six doubles, five homers, 11 RBIs, and a ridiculous 1.351 on-base plus slugging percentage in 11 postseason games.

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With all due respect to Reggie Jackson, Harper is the new Mr. October, baseball’s biggest star on its biggest stage.

Harper is in complete control — of everything. Each pitch of every at-bat. The outcome of games. And especially his emotions.

“If that was me, I would have been high-stepping around the bases and everything,” left fielder Kyle Schwarber said. “He was just calm as can be running around them. It’s really cool, man. That guy is the ultimate bro.”

It wasn’t always this way. Early in his career, as a member of the Washington Nationals, Harper was branded even by some of his peers as “overrated.” A few weeks after Harper’s major league debut, Cole Hamels hit him with a pitch on purpose. Three years later, then-Nationals closer Jonathan Papelbon tried to fight Harper in the dugout over not running out a pop fly.

And when the Nationals won the World Series in 2019, the year after Harper left as a free agent, a preposterous notion arose that his departure somehow made them better. As if a team could actually get better without Bryce Harper.

Some of the criticism could be chalked up to jealousy. Sports Illustrated put Harper on the cover when he was 15 and dubbed him “the most exciting prodigy since LeBron.” That isn’t an easy label to wear when you’re still years away from playing in a major league game.

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But Harper also brought some of it upon himself, with a look-at-me style that rubbed old-school baseball people the wrong way.

”You don’t find a whole lot of people as driven and as confident in himself as Bryce Harper,” said Chad Tracy, Harper’s teammate in 2012. ”Even though he was 19 years old, he was jumping in the mix. Most kids wouldn’t dare say a word in the clubhouse with guys who’ve been around for a while. Not Bryce. He’d chirp in and let everybody know his opinion, and we’d kind of look around going, ‘What’s he doing?’“

Castellanos, who befriended Harper when they were teammates on the under-18 national team at the Pan Am junior world championships in Venezuela, recalled a game three years ago when Harper hit a walk-off grand slam against the Chicago Cubs. He pumped his arms, high-stepped down the first-base line, and sprinted around the bases, shouting and boiling over with emotion.

Castellanos, who was with the Cubs at that time, contrasted that “crazy energetic” reaction with Harper’s composed demeanor after hitting the biggest postseason homer in Phillies history.

“This is how I know he’s grown as a person,” Castellanos said. “Watching him, it was a big lesson for me, being able to watch and learn.”

Harper doesn’t like to look back. He says baseball is about “what you’ve done for me lately,” so he prefers to live in the moment, especially when the moment involves playing in a World Series.

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But there’s also no denying he has grown up. He’s married now, with two children. He didn’t want an opt-out clause in his 13-year, $330 million contract with the Phillies because he wanted to put down roots. He has done exactly that.

“When we’re at home and just hanging out with Kayla and his kids, he’s being a dad,” said rookie shortstop Bryson Stott, who grew up with Harper in Las Vegas and is living with him during the playoffs. “But then he’ll just drop, ‘I can’t wait to play tomorrow.’”

On Sunday, Harper caught a ceremonial first pitch from former Phillies outfielder Jayson Werth. They were teammates in Washington when Harper arrived as a hotshot rookie and Werth as a veteran with a new $126 million contract. It was a protégé-mentor relationship.

Now, though, it was Werth who stood in front of the mound, took a step, and whipped a sidearm fastball at Harper, who shook his head disapprovingly.

“He [ticked] me off because he threw that ball so hard. Golly,” Harper said later. “I had no idea he was throwing it. ‘What are you doing? I’ve got to play a game.’ Thank goodness I’m a catcher, or I used to be. I wanted to kill him. So J-Dub.”

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A few hours later, all that Werth, or any of the other 45,000-plus souls at Citizens Bank Park, could say about the outcome was that it was “so Harper,” for whom nothing matters more this month than winning the championship that has eluded him.

”I think it means everything to him,” Harper’s father, Ron, said. “Besides his family, yes, 100%. I am elated that he’s helped this team that has one heartbeat. They all trust each other and love each other. The city loves him. I couldn’t be more happy for him.”

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Said Stott: “He can’t wait to get back out there each day. He lives for this. Any time you have a superstar that has missed out on the postseason a few years in a row, they just want to get back and want to be on that big stage.”

The stage belongs to Harper. At last.

Join Scott Lauber and Alex Coffey, Phillies beat writers for The Philadelphia Inquirer, as they preview the World Series on Gameday Central, Friday at 6 pm at inquirer.com/PhilliesGameday