KIEV, Ukraine — He’s a quick-witted politician who understands the power of television, has broad public support at home and, perhaps not so important, sometimes takes off his shirt in public.

And he is not President Vladimir V. Putin, Russia’s paramount leader of nearly 20 years.

This is Volodymyr Zelensky, the former comedian who became Ukraine’s president in May and has followed his unconventional rise to power with an equally unexpected display of diplomatic flair.

Aides to Mr. Zelensky insist that his focus is domestic overhaul, and that he is not striving to become the anti-Putin in a region where the Russian leader has faced much criticism but few competitors for star power.

But a rivalry is already apparent. Ukraine’s relationship with Russia, which seized part of its territory in 2014 and has continued to back a wider separatist uprising, is the pivot around which many of Europe’s most pressing security problems revolve. Mr. Zelensky has approached it with a combination of calculated assertiveness and strategic generosity, reaching out to Russian speakers whom his nationalist predecessor could not hope to win over.