Like every other fandom pairing the pairing of Tyson and Rei has more hints than proof, but you don't have to look too hard to find them.

Rei started off as a cocky know it all who, surrenders to Tyson and joins the team, after realizing Tyson was going to be on it. He became instant friends with him as well.

In China however they seem to get even closer, with the White Tigers constantly challenging Rei. Tyson continuously jumps to his defense and even goes as far as saving him from losing when his bitbeast leaves him and battles Kevin in his stead. The next morning Rei leaves and Tyson freaks out, being the first one to rush to find him.

Upon finding him he challenges Rei to get him to join the team again, claiming that whether he has a bitbeast or not doesn't matter. Upon winning Rei decides to rejoin the Blade Breakers, turning his back on the White Tigers and Mariah once more in order to join forces with Tyson. 

Rei is constantly jumping to support Tyson, including doing his best to save him numerous times. He becomes a shoulder to support Tyson, continuing through the season until he faces Bryan. Tyson takes this very seriously and tries to protect Rei, eventually telling him to concentrate on defense.

When Rei was knocked unconcious Tyson rushed to his side and Rei gave Tyson the job of avenging him. Tyson takes it very personally and goes as far as to hold his hand as he had been on the stretcher.

Rei runs to his defense, protecting Tyson during the battle against Jin of the Gale and Daichi and goes out of his way to battle Jin of the Gale, himself, after finding the invitation while skulking around in Tyson's yard. He was there to check on him, and make sure he was happy.

When Rei and Max both leave the team Tyson is very distrubed, more so with Rei. He takes it very personally and even starts a fight with him. He takes the battle against Rei very personally and loses control, blaming Daichi for a loss to Rei.