I doubt that it's Rei. I'm still leading towards the theory that he is Vector and Vector is overpowered enough to where he could separate himself from his human disguise. Even if he isn't, he's clearly a villain, so I doubt that he'll help Yuma out, especially that soon after the duel with Vector. Durbe is more of a possibility since they've already revealed his human form in a magazine ad and they usually do that not too long before they appear in the anime. I remember that they showed Dr. Faker's ridiculous Barian form shortly before his duel and then Vector's Barian form just a few weeks before he appeared. It could be a new character too, but at this point, I'm leading a bit towards Durbe.