Who is Viktor Medvedchuk and why does his arrest matter to the Kremlin?

Analysis: Putin's ally was Russia's main guy in Ukraine' but Kremlin has refused prisoner exchange

He was Vladimir Putin's best friend in Ukraine but the Kremlin says it won't make a trade to secure his freedom.

Viktor Medvedchuk, a man often named the dark prince" of Ukrainian politics, has been a loyal ally to Putin for two decades, leading a pro-Russian political party in Ukraine for years, even after the Russian annexation of Crimea in 2014 and the beginning of the deadly war in Donbas.