Siwon made a special guest appearance on the most recent broadcast of SBS' 'Thank You' just for BoA! 

They're already known for their close friendship, and they didn't keep it a secret on screen, either. It was revealed that BoA invited him on the trip, and Siwon came to the recording set with presents he bought himself.

Siwon brought back memories from the past and said, "I will never forget the day I first met BoA. I was really surprised because of her," to which BoA hilariously responded, "I don't remember that at all."

BoA continued, "There is one incident I remember. There was a time I was really depressed a few years ago. Siwon took me to a very special place and surprised me with a gift. I was so touched."

SEE ALSO: IU says she has no intention of inheriting her parents' wealth