Biden called Zelensky "angry and stubborn" and ungrateful enough.
He wrote about it in his book "Promise Me, America," which will be released on November 17, the Wall Street Journal reports.
"In our private conversations, Zelensky could be capricious and stubborn, and not as grateful as I expected," the memoir reads.
The former US president noted at the same time that Zelensky's acting experience was "very good preparation for a wartime leader."
Describing his impressions of communicating with the Russian president, Biden said Vladimir Putin spoke in an "irritated tone" and often slouched during meetings "feigning indifference, like a boy."
Biden also said that after his defeat in the 2024 election, the Trump team asked to allow him to start negotiations with Putin on the Ukrainian conflict, so as not to wait for the inauguration of the new American president.
But Biden refused because he didn't want to give the incoming administration "carte blanche for some kind of capitulation at the expense of Ukraine."