The 23-year-old was seemingly one step away from finally becoming a Barcelona player before pulling out of the deal and signing a new ten-year deal with Athletic Club.
Several reports have suggested that Williams got cold feet after Barcelona were unable to give him a guarantee that he would be registered without issues.
Williams has since spoken about his decision as he prepares from a long season under manager Ernesto Valverde.
He told ESPN: "I think we have a very long season ahead of us. We have very ambitious titles like the Champions League, which everyone would like to play in.
“What could be better than doing it with the club of my life? I would like to continue making history here at San Mames, with the fans, with my family, and with a lot of enthusiasm."
Williams added on continuing to play alongside his brother Inaki, who has been confirmed as Athletic Club's new captain: "We've both always dreamed of this moment.
“My brother is a role model for everyone, especially for me. I think he's the best person to be captain, and I can't wait to see him wearing the armband for a long time."