Jiro Nagasawa (長沢 二郎, Nagasawa Jirō; February 2, 1932 – March 23, 2010) was a Japanese swimmer. He is credited with the invention of the modern butterfly stroke. Although not the first to swim the butterfly-breaststroke, Nagasawa was the first to use a dolphin kick, where the legs stayed together and moved simultaneously up then down.
Source: “Jiro Nagasawa”, English Wikipedia (CC BY-SA 4.0), excerpt