Here's why:
* They are rivals: Shun and Masquerade are fierce rivals. Masquerade is the main antagonist in the first Bakugan season, and their battles are some of the most intense.
* Masquerade's true identity: Masquerade is actually Dan Kuso's father, who is obsessed with the power of the Bakugan and seeks to control them. He uses his son's best friend, Shun, as a pawn to further his own goals.
* Different sides: Shun is a strong, loyal Bakugan fighter who believes in justice. Masquerade, on the other hand, is consumed by ambition and power. Their ideologies are completely opposed.
While they have a close relationship through their rivalry, it's not a partnership. It's a constant battle of wills and ideologies.