The Kremlin official boasted of his commitment to healthy living, opening a door in his office to show a visiting businessman what looked like a private gym. Then he described his latest project: stage-managing “referendums” in occupied Ukraine to make it look like those regions wanted to join Russia.

The Moscow businessman, who had come to see him about another matter, recalled that the official, Sergei V. Kiriyenko, had gone into great detail about the referendums, even listing the percentage breakdown of the results the Kremlin would declare.

He added that Mr. Kiriyenko left the impression of a calm, ambitious bureaucrat “solving a concrete, technical problem.”

Since that meeting three years ago, it has become more clear than ever that Mr. Kiriyenko is the man who turns President Vladimir V. Putin’s ideas into action.

As the Russian leader wages war, Mr. Kiriyenko oversees wide-ranging government efforts to tighten Mr. Putin’s grip on the country and on occupied Ukraine. He has also recently gained new power inside the Kremlin, taking over much of the portfolio of another Putin aide who disagreed with the invasion of Ukraine.

Despite his modest title of first deputy chief of staff to Mr. Putin, Mr. Kiriyenko represents an underappreciated aspect of how the Russian president exercises power, forming part of a cadre of skilled, loyal and opportunistic managers who direct the sprawling apparatus of the Russian state.

For more than three years, Mr. Putin has leaned on Mr. Kiriyenko, 63, to manage the political aspects of the Ukraine war. Cracking down on domestic opposition. Expanding the Kremlin’s control of the internet. Pushing Mr. Putin’s narrative into Russian schools and culture. Shaping propaganda and governance in occupied Ukraine. Attempting to legitimize Russia’s land grab.

Just in the past few months, Mr. Kiriyenko’s reach has extended to efforts to reintegrate Ukraine war veterans into civilian life and to push Russians onto a state-affiliated messaging app instead of Western ones. If Mr. Putin makes a deal with President Trump at their planned summit in Alaska on Friday to end the fighting in Ukraine, it is likely to be Mr. Kiriyenko’s job to sell any compromise to Russians as a victory.

In interviews, more than a dozen former colleagues and other Russians who know Mr. Kiriyenko described him as a man whose proficiency in the minutiae of control and influence have greased the machinery of Mr. Putin’s autocracy. Many of the people, including three close to the Kremlin, spoke to The New York Times on the condition of anonymity for fear of retribution.

The Kremlin declined to make Mr. Kiriyenko available for an interview and did not respond to a request for comment.

One of his former aides, Boris B. Nadezhdin, said that he noticed Mr. Kiriyenko’s skill at managing personnel and at staying in his bosses’ good graces three decades ago, when Mr. Kiriyenko was a deputy energy minister. The two men would collide in 2024, when the Kremlin blocked Mr. Nadezhdin’s attempt to run for president against Mr. Putin.

Mr. Nadezhdin noted in an interview that Russia’s era of independent politicians had passed. He said that the Putin era belonged to those like Mr. Kiriyenko — “a person who does not try to implement any of his own plans, ideas and so on, but simply, clearly carries out tasks.”

‘Without Rules’

Mr. Kiriyenko casts himself as a student of the cold calculus of power.

He is a sixth-rank black belt in aikido, a Japanese martial art focused on harnessing an opponent’s energy and turning it against them. He professes an interest in Methodology, a Soviet-era school of philosophy in which society can be engineered, managed and transformed from above.

In the tumult of modern Russian politics, that focus on power has translated for Mr. Kiriyenko into shifting alliances and repeated reinvention. “In a game without rules,” he once told an interviewer, “the one who makes the rules wins.”

Mr. Kiriyenko was just 35 in 1998 when he briefly became Russia’s prime minister. His youthful image and meteoric rise — he’d been a regional oil refinery manager a few years before — earned him the nickname Kinder Surprise, a play on the name of a European children’s candy.

After losing his post when Russia defaulted on its debt, Mr. Kiriyenko co-founded a party pushing Western-style economic overhauls. He took a crash course in literature to appeal to the urban middle class, reading five books a week in the midst of his 1999 election campaigns for Moscow mayor and for the Russian Parliament, according to Marat A. Guelman, then his campaign manager.

“He was quick to perceive, quick to change,” said Mr. Guelman, who later turned against Mr. Putin and now lives in Berlin.

After Mr. Putin won the presidency in 2000, Mr. Kiriyenko pivoted again and quit Parliament to work for the Kremlin. A few years on, Mr. Guelman asked for help for an associate who had run afoul of the authorities, describing him to Mr. Kiriyenko as “a person of our convictions.” Mr. Kiriyenko, Mr. Guelman recalled, shot back: “I don’t have convictions now — I’m a soldier of Putin.”

Alfred R. Kokh, a 1990s-era deputy prime minister of Russia who also left the country, described a similar exchange. He complained to Mr. Kiriyenko in 2003 about improprieties in that year’s parliamentary election campaign.

“Are we going to la-la,” Mr. Kiriyenko replied, “or are we going to talk business?”

Powerful Friends

Already ensconced in the Kremlin machinery, Mr. Kiriyenko ran one of the government’s biggest businesses from 2005 to 2016: Rosatom, the state nuclear energy conglomerate.

During those years, Mr. Kiriyenko deepened a bond with a banking and media magnate, Yuri V. Kovalchuk, according to Western officials and several of the Kiriyenko associates who spoke to The Times. A physicist by training, Mr. Kovalchuk is widely seen as one of Mr. Putin’s closest friends.

He persuaded Mr. Putin to bring Mr. Kiriyenko back to the Kremlin, some of those people said. Mr. Kiriyenko had proven himself at Rosatom, modernizing the company with Japanese management principles and extending Russian influence by striking deals around the globe.

In his new Kremlin job, Mr. Kiriyenko was entrusted with orchestrating Mr. Putin’s version of democracy, an exercise in cementing the president’s legitimacy and keeping control of a far-flung nation. As the first deputy chief of staff overseeing domestic politics, Mr. Kiriyenko planned the selection of the Kremlin’s preferred candidate for governor in each of Russia’s more than 80 regions, the elections to fill the more than 600 seats in Parliament and the stage management of Mr. Putin’s own re-election in 2018 and in 2024.

“He’s the technical implementer,” said Grigory A. Yavlinsky, a liberal politician in Moscow who ran for president, with the Kremlin’s approval, in 2018. “It’s a huge amount of work.”

Mr. Kiriyenko also held contests to identify the next generations of technocrats, featuring online aptitude tests and role-playing leadership games. Just this year, finalists of his “Leaders of Russia” competition have been named to government roles such as auditing construction projects in occupied Ukraine, managing bus transit in suburban Moscow and running the health ministry in Khabarovsk in Russia’s Far East.

He has broadened his portfolio further by taking on Russia’s last bastion of free speech: the internet. In 2021, Mr. Kiriyenko wrested control of the country’s most popular social network, VK, from an oligarch. Mr. Kovalchuk put up much of the money. Mr. Kiriyenko’s son became C.E.O. Mr. Kovalchuk’s grandnephew took another senior role.

The power of that alliance was on display in a blitz that many analysts saw as a prelude to a potential ban on WhatsApp.

In March, VK unveiled its own messaging app. In June, Russia’s communications minister praised the company for releasing a “fully Russian messenger” in a televised meeting with Mr. Putin. Days later, Russian lawmakers passed a bill mandating that a Russian-made messaging app should come preinstalled on all smartphones. In July, the government announced that this app would be the one developed by VK.

“For us, the government is always a partner and a senior comrade,” Mr. Kiriyenko’s son and the head of VK, Vladimir S. Kiriyenko, said in April.

Backing the Invasion

As Mr. Putin massed troops and plotted his 2022 invasion of Ukraine, the president’s political aides were largely in the dark, Mr. Kiriyenko’s associates said. The three people close to the Kremlin said they were convinced that Mr. Kiriyenko didn’t share the fixation on Ukraine’s pro-Western turn that drove Mr. Putin to attack the country.

After the war started, Mr. Kiriyenko soon refashioned himself once again. Trading his suit for olive-green shirts, he started traveling to occupied Ukraine amid the fighting, touring hospitals and schools.

He worked on planning a public “war crimes” trial of Ukrainians to show Mr. Putin fulfilling his promise to “denazify” the country, one of his associates told The Times in June 2022. The trial never materialized as Russian forces struggled on the battlefield, but Mr. Kiriyenko said at a conference in 2023 that the war “must end with trials of Ukrainian criminals.”

He did succeed in putting on a different show — the sham referendums in which Moscow claimed Ukrainians under Russian occupation had voted overwhelmingly to become part of Russia.

Inside Russia, Mr. Kiriyenko used the levers of his office to try to engineer popular support for Mr. Putin’s invasion.

The Public Projects Directorate, a unit focused on patriotic initiatives that Mr. Kiriyenko oversees, developed propaganda lessons for Russian schoolchildren. His staff also pressured midlevel officials to serve stints as administrators in occupied Ukraine, said Sergei Markov, a pro-Putin analyst in Moscow who has worked with the Kremlin.

“Sure, those who don’t want to can refuse,” Mr. Markov said. “But in that case they understand that they’ll face serious limits on their careers.”

Mr. Kiriyenko’s portfolio also includes the arts. He has ramped up government support for pro-war entertainers who backed the war while blackballing those critical of it, according to Russian media reports. Iosif I. Prigozhin, a major music producer, said in an interview with The Times that the Kremlin gave “a blank check” after the invasion to musicians who were “more focused on national interests.”

Mr. Prigozhin’s wife, the pop star Valeria, has performed at patriotic concerts in Red Square. He called Mr. Kiriyenko “positive, decent, sensitive and precise.” When Mr. Kiriyenko’s office seeks performers for events, “the approach is not demanding, but suggestive,” Mr. Prigozhin said.

Mr. Kiriyenko’s policies are also backed up by the full force of the Russian state. Thousands of antiwar Russians have been prosecuted or forced into exile in an effort that many analysts, opposition figures and the former colleagues of Mr. Kiriyenko say they believe was largely coordinated by him as the Kremlin official who oversees domestic politics.

Ilya V. Yashin, a Russian opposition leader, had just been arrested and interrogated in July 2022 when he said he chatted with a security service agent in the grim corridor of a law enforcement agency in Moscow while waiting for his prisoner transport to arrive.

The agent told him that his arrest was a “political decision,” dropping hints about a “Sergei” in the Kremlin who was a “buddy” of Boris Y. Nemtsov, the politician who brought Mr. Kiriyenko into government in the 1990s. The suggestion was that Mr. Kiriyenko was responsible for his fate, Mr. Yashin recalled in an interview after his release in a prisoner exchange last year, though he noted he couldn’t be certain of Mr. Kiriyenko’s role, if any.

To Mr. Yashin, the irony was remarkable. Both he and Mr. Kiriyenko were allies, at different times, of Mr. Nemtsov, a Russian opposition leader assassinated in 2015.

“Now Nemtsov is dead, and one of his friends put another one in prison,” Mr. Yashin wrote from jail in 2022.

‘Absolutely Opportunistic’

In February of this year, Russian state news outlets reported that Mr. Kiriyenko was managing public unrest in Abkhazia, a Russian-backed breakaway region of Georgia. To help show the benefits of being on the Kremlin’s side, Mr. Kiriyenko offered a gift of 20 Russian school buses and organized a version of his trademark leadership competitions.

Mr. Kiriyenko’s remit has been increasingly expanding outside Russia’s borders.

A different Kremlin deputy chief of staff, Dmitri N. Kozak, oversaw relations with Abkhazia as recently as last year. But Mr. Kozak has lost influence in Moscow amid his criticism of the invasion of Ukraine, according to the three people close to the Kremlin, a U.S. official and a Western contact. In the past few months, they said, Mr. Kozak presented Mr. Putin with a proposal to immediately stop the fighting in Ukraine, start peace negotiations and reduce the power of Russia’s security services.

The Russian president has kept Mr. Kozak, who has been at Mr. Putin’s side since the 1990s, in his senior post. But he has shifted much of Mr. Kozak’s portfolio to Mr. Kiriyenko, including managing Kremlin relations with Moldova and with the two breakaway regions of Georgia, the people said.

The expansion of Mr. Kiriyenko’s influence shows how his star continues to rise at the Kremlin as he embraces and executes Mr. Putin’s wartime policies.

Mr. Kiriyenko is “effective” and “absolutely opportunistic,” Mr. Yashin said. If Mr. Putin or a future Russian leader pivots back toward the West someday, Mr. Yashin said, “Kiriyenko will find the words for it.”

Paul Sonne, Michael Schwirtz and Julian E. Barnes contributed reporting.