The Pikachu fighters' costumes represent the Palicos, a race of anthropomorphic cats who wear different helms and serve as the player's companions.
The stage represents the Buddy Plaza, a forest-like location in Monster Hunter Rise where the Palicoes can be hired to the player's team.
The Pikachus starting off with a Boomerang, a Bob-omb, and a Pitfall reference the Palicoes' usage of weapons, including a boomerang, a bomb, and shock traps.
Other trivia:
This spirit's Support type references the Palico's role in Monster Hunter Rise as the player's companions and usage of support attacks.
This spirit's Ramblin' Evil Mushroom Equipped ability references how Mushrooms can be used to craft weapons and armor in Monster Hunter Rise.
