The campaigning for the first round of the presidential election in Ukraine has just ended. It’s March 30 and I’m in a seafood restaurant near St. Sophia’s Cathedral with a small man in a black T-shirt, his voice a bit hoarse, overflowing with high-octane energy. Volodymyr Zelensky is still a little-known comedian. He will shock the world tomorrow by coming in far ahead in the first round of voting.

I tell my dinner companion right off the bat that I’m a friend of incumbent President Poroshenko, that I met with him the night before, and that I believe Mr. Poroshenko—who would finish second in the first round, setting up an April 21 runoff—deserves credit for having built an army, kept Odessa and Mariupol out of Russian hands, and enabled Ukraine to escape the grip of debt and recession.

He cuts me off. “I know, good for you. But all that’s ancient history. And I’m the one sitting across from you now.”

It was Mr. Poroshenko, Mr. Zelensky tells me, who helped give him the idea of going into politics by asking him to join his team last year.

He also had conversations with his friend Ivan Vacarcuk, the Bono of Ukraine, who had long considered running for office himself before giving up the idea and, in a sense, passing the torch to Mr. Zelensky.

But, he told me, it was the talks with Mr. Poroshenko—talks in which Ukraine’s president pleaded with Mr. Zelensky to “lend us your name” without ever asking about his beliefs or convictions—that decided the matter.

All right, I say. But now where do you stand? There are three possible models. The best case is Reagan; the worst is Italian Beppe Grillo; and, in between, there’s Coluche. Mr. Zelensky is familiar with the first two. My mention of Mr. Grillo and his compromise with Italy’s far right seems to disgust him.

He isn’t familiar with the French comedian Coluche and is surprised when I tell him that Coluche was riding very high in the French presidential polls in 1980 when he suddenly withdrew.

“Withdrew? Why would he do that?”

He had wanted to demonstrate that the emperor had no clothes and should go back and get dressed, I explained. He had accomplished that, and so he had too much respect for politics to stay in the race.

“OK, I get it,” Mr. Zelensky said. “But that was in François Mitterrand’s time, wasn’t it? We’re in Ukraine, and we don’t have a François Mitterrand.”

I lack the space here to report in detail the long conversation that ensued. But it turned on four major themes.

First, Vladimir Putin—his resolute opposition to Mr. Putin. “The guy has no look; he has eyes, but no look.” Mr. Zelensky wants badly to meet the Russian leader face to face. Why? “You realize that I can make even Putin laugh? A hollow laugh, but a laugh all the same. And all those Russian young people who know me very well will burst out laughing with me! What did you say about Coluche? The emperor has no clothes?”

Second, his program. Now he breaks into laughter when I object that you need a plan to govern and that he, as far as I know, doesn’t have one. “Oh? You think that, too? Well, that’s your problem, my friend, not mine. Because my program exists, and it’s public. Except nobody’s bothering to read it. As for my team, tell your journalist and diplomat friends that they’re in for a big surprise. . . . Since you all see me as a showman, it’ll be the best show of the campaign and of my entire life.”

Third, his Judaism. It is extraordinary that the possible future president of the country of the “Holocaust by bullets” and Babi Yar is an unembarrassed Jew from a family of survivors from Kryvyi Rih, in Dnipropetrovsk Oblast—a land with a history of pogroms. Is this postmodern kid proof of the decline of anti-Semitism since the 2014 revolution in the Maidan? Could his election be seen as an act of collective repentance by the Ukraine of Nestor Makhno and Stepan Bandera? Mr. Zelensky is clear: “The fact that I’m Jewish ranks about 20th on the long list of my features.”

Fourth, French President Emmanuel Macron. As we were getting up to go, Mr. Zelensky cupped his hand and said, mimicking my French accent: “That guy you were talking about? Coluche? I have nothing to do with your Coluche or any of the others. The person I admire is Emmanuel Macron. Plus, we were born the same year! Please give him this message: I wouldn’t mind taking a break between the two rounds to visit the Eiffel Tower again.”

I left feeling troubled and a little sad. Volodymyr Zelensky is more than his caricature and may not be the populist I had criticized the day before, in a speech at Taras Shevchenko University.

But I’m sad for Petro Poroshenko and haven’t resigned myself to his loss. The man who stood up to Mr. Putin—the commander in chief I accompanied in Kramatorsk, at the bedsides of the victims of one of the most indiscriminate bombings committed by pro-Russian separatists, the unlikely but valiant giant whom I watched, in the depths of the Ukrainian winter, confront adversity and solitude—deserves better than to be fired at history’s whim.

But I’m fairly sure we haven’t heard the last from him. The man I brought to the Élysée in March 2014, when he was in limbo in the polls, has not said his last word. For Ukraine and for Europe, he must win.