Dokgo Tak - Throw to the Sun (Golden Arm) (1983)

Hun-yi and Nan-yi are siblings who grow up as best friends. Hun-yi is a naturally talented pitcher and joins a baseball club. Under the guidance of a professional baseball player, the children learn to play baseball, but they don't want to do any of the hard training required. After a twist, Hun-yi's team makes the finals of the Little Baseball Championship.