I do not know if this holds true for the manga, but in the anime everyone plays the game and has a signature monster. It may be harder to pick one out for the sidekicks/designated cheering section, but they all have one. The only character that we never see dueling is Mokuba Kaiba, but the video games (which may be non-canon) show him using animal monster cards, so one of the best in there would be a good choice.