Killian Tillie was born on March 5, 1998 in Paris, France. The son of Laurent Tillie and Caroline Keulen-Tillie, both of Killian's parents were professional volleyball players. Killian has two brothers: Kim, a professional basketball player; and Kevin, a professional volleyball player. Tillie played basketball and volleyball until the age of 14, when he began to focus exclusively on the former. Hayes joined the youth academy of Olympique d'Antibes in 2013 and later joined the INSEP academy. Tillie also appeared for the French U16 National Team in the 2014 FIBA Championships, averaging 14.3 points, 9.6 rebounds and 1.7 assists per game en route to MVP honors. In 2015, Tillie competed in the FIBA U18 Championships, helping lead France to an 8-1 record and a third-place finish.