Rei is a pretty cool fighter who makes use of magic to augment her abilities. Ryoma can augment his abilities with the Pinnacle of Perfection techniques though. In this form he can make any tennis shot that you can imagine. While that may not sound super impressive on its own you have to factor in how powerful his shots are. He can cause natural disasters with his tennis moves and his serves are so fast that Rei would be unable to react to them. It’d be game over for her. Ryoma wins.
Update 8 7 2020 Rei’s magical powers will be too much for Ryoma. Rei wins.