That "guy" is Hifumi Abe, 26, who claimed back-to-back Olympic titles in the men’s 66-kilogram judo division at the Paris Olympics on July 28. Abe is aware of his obsessively competitive nature.

When Tokyo won the bid to host the Summer Games in 2013, a young Hifumi Abe and his younger sister, Uta, were watching the news on TV along with their family. “It would be great if both of you ...