Xabier Castillo Aranburu is a Spanish footballer who plays for Athletic Bilbao as a left defender. Even though Castillo came up the ranks with Athletic Bilbao, it was with Real Sociedad that he made his senior debut. He first played professionally when he was on loan to the UD Las Palmas in 2006-2007's second division. He soon became the undisputed first choice for the team's main squad during the first two campaigns. On 15 July 2009, he signed a four year deal with Athletic Bilbao and appeared in a sum total of 45 official games. Unfortuanately for Castillo, he had to spent the entire 2011–12 season on the sidelines, due to a patellar ligament injury.