A grandpa who knew Katahara --- Kure Erioh would fit the bill. I'm betting on a chance this is memory loss trope, and it means Ryuuki is no other than Ohma himself who had waken up from coma. His memory might be hazy due to long sleep and brain damage caused by Advance, so grandpa Kure sent him to enter Kengan matches in hope he can recover through the heat of fight.