Max is Greiger's brother. He is part of a tribe that descended from those who served the People of the Stars. It is implied that he and his sister Annie live in poverty, since Greiger is working to get aid for their hometown.
When Yusei and Jack visited the Nazca region, he showed his Deck to Jack, who noticed that he is using a Power Deck, which has some cards that Jack also has in his Deck. Later they had a Turbo Duel, in which Jack was struck by power of his "Red Dragon Archfiend", because Max was being possessed by the Familiar, who used a power reversal strategy.
In reality, Max has only one Deck (unknown if it is supposed to be a Ground Deck or Riding Deck, but assumed to be Ground because of his age) that is similar to Jack's Ground Deck, including many of Jack's cards. When possessed by the Servant of the "Crimson Devil," his true Deck converted over to a different one which focuses on anti-power through Special Summoning and altering statistics.