Ukrainian tycoon Viktor Medvedchuk, one of 56 prisoners handed over to Russia on Wednesday in exchange for 215 detained soldiers, was seen as President Vladimir Putin’s best ally in Kyiv and for years defended the Kremlin’s interests.

The swap was the largest exchange between the warring factions since the Russian invasion began in February.

“In exchange for the life and freedom of our defenders, Ukraine gave away Medvedchuk,” Ukraine’s chief prosecutor Andriy Kostin said in a Facebook message on Thursday.

Denis Pushilin, a leader of the Moscow-backed separatists in eastern Ukraine, confirmed Medvedchuk’s release to the Russian state news agency RIA Novosti.

Medvedchuk, a 68-year-old former Ukrainian MP, was arrested by Ukrainian special services in April after fleeing house arrest before Russia invaded.

The super-rich ruler, once dubbed the “dark prince” of Ukrainian politics, has been accused of treason and attempted theft of natural resources in Russia-annexed Crimea, as well as leaking Ukrainian military secrets to Moscow.

“The pre-trial investigation is complete. Medvedchuk’s testimonies are documented. We don’t release anyone, we move them from a Ukrainian prison to a big prison called Russia,” said Kostin, who was operating from the shadows.

News of the capture of Medvedchuk, who was ranked by Forbes last year as Ukraine’s 12th richest person and widely vilified for his close ties to the Kremlin, sparked celebrations among Ukrainians online.

Medvedchuk and the Kremlin denied that he pulled the strings for Moscow in Kyiv, but the businessman made no secret of his closeness to Putin.

Links between the two men date back to the early 2000s, and Medvedchuk said the Russian leader is godfather to his youngest daughter Darya.

Medvedchuk and Putin have been regularly photographed together at lavish events, including the Formula 1 race in Sochi.

“We have a great relationship. It was built up over many years,” he said in a 2019 interview with AFP.

Kyiv authorities certainly believed that Medvedchuk remained a key Kremlin asset.

Ukraine’s security chief at the time, Ivan Bakanov, said Russia’s FSB secret service was trying to get him out of Ukraine.

And Ukrainian President Volodymyr Zelenskyy suggested exchanging Medvedchuk for captured Ukrainian soldiers – although the idea was rejected by Moscow.

“He is a foreign politician,” Putin’s spokesman Dmitry Peskov said in April.

Medvedchuk, a former lawyer, has long been at the center of Ukraine’s murky connection between money and politics.

The country’s second president’s chief of staff, Leonid Kuchma, has been accused of playing a key role in attempts to rig a 2004 vote in favor of pro-Russian candidate Viktor Yanukovych.

This election sparked an uprising known as the Orange Revolution. The uprising heralded the eventual Maidan revolution that tipped Ukraine westward in 2014 and ousted Yanukovych from power.

Medvedchuk continued to operate in the shadows during the turmoil that shook his homeland, as Moscow responded by seizing the Crimean Peninsula and igniting a war in eastern Ukraine.

He has been sanctioned by the United States for undermining the government and has been involved as a facilitator in peace talks and prisoner exchange negotiations.

Eventually he returned to the center of the political stage.

In the 2019 general election, he led the Moscow-backed opposition Platform for Life party, which finished second to Zelenskyy’s bloc.

But Medvedchuk’s fortunes started to take a downturn in February 2021.

Zelenskyi banned three pro-Russian TV channels he was associated with, and authorities then seized his family’s assets, including a pipeline carrying Russian oil to Europe.

In May of that year he was charged with “treason” for attempting to steal assets from Russia-annexed Crimea and later also for attempting to buy coal from separatist-held regions.

He denied the allegations but was placed under house arrest.

Putin denounced the crackdown on him as a “political” purge and vowed to “react.”

“Medvedchuk was Putin’s deputy, his most trusted man, his eyes and ears in Ukraine, broadcasting messages from Moscow through his media,” said former lawmaker Sergiy Leshchenko.

In January, as fears of a Russian invasion mounted, the US accused Medvedchuk of complicity in efforts by Russian intelligence agencies to prepare friendly Ukrainian politicians to take control of the country with the backing of the occupying powers.

Days after Russian troops rolled over the border, Ukrainian police announced that Medvedchuk was missing after officers failed to locate him during a search of his lavish home near Kyiv.

After his escape, Ukrainian media ashore at his residence spotted a large wagon decorated with gold and velvet and the Russian coat of arms, apparently a birthday present from his wife, TV presenter Oksana Marchenko, who had fled to Russia.