When you dig into how the Kremlin wields power, one name comes up over and over: Sergei V. Kiriyenko.

Mr. Kiriyenko, 63, is a first deputy chief of staff to President Vladimir V. Putin. The modest title belies enormous influence. He is the key figure who tightened Mr. Putin’s grip on Russia since the full-scale invasion of Ukraine, even though Mr. Kiriyenko was once known as a Western-oriented reformer.

Here are the key takeaways from our yearlong reporting on Mr. Kiriyenko, based on interviews with more than a dozen former colleagues and other Russians who know him, as well as with Western officials.

A Consummate Technocrat

Mr. Kiriyenko’s importance to Mr. Putin is as one of his most effective and versatile “technocrats.” These men and women are an underappreciated reason Mr. Putin has been able to rule for a quarter-century. Younger and more internationally minded than the K.G.B.-era figures closest to Mr. Putin, the technocrats have become proficient in managing Russia’s finances, social programs, health system, infrastructure and many other crucial functions.

Mr. Kiriyenko ran Russia’s state nuclear energy conglomerate for a decade, modernizing it with Japanese management principles and expanding its footprint with global deals. Since 2016, he has managed Russia’s domestic politics, including overseeing regional governments, picking the Kremlin’s favored candidates for Parliament and helping coordinate a crackdown on dissent.

His portfolio keeps growing. He’s now spearheading the Kremlin’s push to control the internet, working to indoctrinate schoolchildren with Mr. Putin’s ideology, overseeing governance of occupied Ukraine and managing relations with parts of the former Soviet Union, like Moldova and the breakaway regions of Georgia.

Strategic Partnership

One foundation of Mr. Kiriyenko’s power, our reporting shows, is his relationship with one of Mr. Putin’s closest friends, Yuri V. Kovalchuk.

Mr. Kovalchuk, a media and banking magnate, is a physicist by training. Mr. Kiriyenko grew closer to him while running the state nuclear company, Rosatom. Mr. Kovalchuk then persuaded the Russian leader to bring Mr. Kiriyenko back into the Kremlin. Mr. Kovalchuk and Mr. Kiriyenko have worked together to accelerate Mr. Putin’s push toward autocracy.

In 2021, Mr. Kiriyenko and Mr. Kovalchuk joined forces to take over VK, Russia’s biggest social network. This summer, Mr. Putin signed a law to require smartphones to come preinstalled with a messaging app developed by VK — a move seen as a prelude to a ban on WhatsApp.

Embracing the War

In interview after interview, Mr. Kiriyenko’s associates and former colleagues emphasized his ideological flexibility. In the 1990s, when he briefly served as prime minister, he was seen as a pro-Western reformer. Then he became an architect of Mr. Putin’s authoritarian system. Now he is a chief enabler of Mr. Putin’s war against Ukraine.

Mr. Kiriyenko and other Kremlin political aides were out of the loop as Mr. Putin planned his 2022 invasion of Ukraine, according to our reporting. But as Russian missiles rained on Kyiv, shocking much of Moscow’s ruling elite, Mr. Kiriyenko embraced the war.

That set him apart from an even longer-serving Putin aide, Dmitri N. Kozak, who had previously overseen the Kremlin’s relationship with Ukraine. Mr. Kozak made it clear that he considered the invasion a mistake and has since advised Mr. Putin to compromise and end the war, our reporting shows.

Mr. Kozak’s standing with the president has fallen as a result, while Mr. Kiriyenko’s power has grown. Mr. Kiriyenko emerged as the Kremlin’s envoy to occupied Ukraine. He planned the sham referendums in which Moscow claimed that Ukrainians under Russian occupation had voted to become part of Russia. And he has taken over much of Mr. Kozak’s portfolio of dealing with other former Soviet countries.

Range of Tools

Mr. Kiriyenko has shown himself proficient — and even creative — at wielding the vast powers of the state to serve his boss.

He leveraged his oversight of culture and education to push performers to produce more patriotic content and to organize pro-war propaganda in Russian schools. He used his oversight of regional governments to direct administrators to serve stints in occupied Ukraine. He expanded leadership competitions that aim to foster the next generation of technocrats.

Given the Kremlin’s control over the judiciary, Mr. Kiriyenko worked on planning a public “war crimes” trial of Ukrainians to show Mr. Putin fulfilling his promise to “denazify” Ukraine. The trial never materialized, however, as Russian forces struggled on the battlefield.

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