Courtois, whose team won 4-0 at Stoke on Saturday, said: “I have been with the guy for six years and he is a very good friend of mine.
“Diego was under-rated in England. People put too much attention on him being a little bully on the field rather than looking at his quality.
“He is one of the nicest, funniest guys I know."
Chelsea are at Atletico this week for their Champions League clash and Courtois is excited to be returning to his former club.
He added: “It will be special to go back, only this time I hope we win.
“It will be different from going to the Vicente Calderon but they needed a new stadium.
“It will be a nice evening for me and I will be very hungry and focused on getting three points.