Courtois, who had less than a year left on his Chelsea contract, wanted to move to Spain for personal reasons.
Kepa said he became aware of Chelsea's interest in signing him to be the replacement for new Real Madrid goalkeeper Thibaut Courtois several weeks ago, and that Athletic had rebuffed several offers from the club before his buyout clause was met on Wednesday.
The deal that sees Courtois join Real Madrid for a reported £35million sees midfielder Mateo Kovacic head the other way on a season-long loan.
Belgium worldwide goalkeeper Courtois is poised to leave for the Bernabeu, with the news that Chelsea have activated Kepa Arrizabalaga's release clause freeing up the 26-year-old to leave.
The two clubs reached a deal reportedly worth 80 million euros and the 23-year-old Spaniard signed a seven-year contract with the 2017 English Premier League champion.
His arrival will surely spell the end for some of his Chelsea teammates time at the club, as he pushes them further down the pecking order right before tomorrow's transfer window closes.
"When I [first] saw him play, I was just so struck by the fact that a keeper who was then only 18 could perform like that under such pressure". Chelsea has made a great bet in taking me in.
"It's very courageous of the club to pay for me".
"A huge thank you to all my team-mates and the technical staff, who I shared success and wonderful moments with". I've analysed what is best for me so I consider the opportunity presented to me in signing for Chelsea is the best for my future.
"I know some of the players who are in the current Chelsea team because I played with them in the Spanish national team".
"I hope they remember me fondly, and I wish them all the best".
"I want to thank Athletic for the love and respect they have instilled in me during these years and I'll always take that with me".