As a boy growing up in a suburb of Chicago, the future Pope Leo XIV did not pretend to be a cowboy or a bank robber. Instead, he liked to play priest, according to his eldest brother, Louis Prevost.

“We teased him a lot about, ‘Na na na, you’re gonna be the pope,’” Mr. Prevost, 73, recalled in an interview on Friday at his home in Port Charlotte, Fla.

But it came as a bit of a shock when Robert Francis Prevost — Rob to his family — was in fact elected to lead the world’s 1.4 billion Roman Catholics.