The 18 year-old Japan international will join Real after the Copa America from FC Tokyo.
Kubo had been with Barca since he was eleven years of age before he was forced to return to Japan after a FIFA crackdown on the recruitment of junior players.
"He was one of the best players in our home base and probably the best of recent years. A player with a very clear profile for the first team," said a Barca coach who worked very closely with Kubo.
"And the worst thing I know is that he is now very good," he added to AS.
"He had the Blaugrana DNA fully integrated into his game, he was one of ours."
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!