Ken Griffey Jr. is a former Major League Baseball player who played for the Seattle Mariners, Cincinnati Reds, and Chicago White Sox. He was inducted into the Baseball Hall of Fame in 2016 and is widely considered one of the greatest players of all time. His nickname was 'The Kid' and his full name is George Kenneth Griffey Jr. The answer to the crossword puzzle is KEN, which is the first three letters of his first name.