Could he be Kiruma Tatsuki? The facial features and hair match, and he bet his life (and lost) in the past. We did see him get cremated though... just putting it out there, the returning king may not be Souichi but Tatsuki (by some plot contrivance), which utterly shocked the referees present.
Think maybe there's an ulterior motive for this fight? It is presumably the end of the manga and we know absolutely nothing about Baku. Perhaps that is by design and he is meant to be a blank slate, but his behaviour towards Hal may not be limited only to him wanting the bet to surpass all bets.