Boktai’s Japanese homepage has revealed some additional details about multiplayer modes. By linking up GBAs, up to four players can battle each other with the parts they earn from the game. The sunlight sensor affects how fast each player is able to charge his or her weapons. Depending on the results and performance of each battle, players are rewarded with items, which can be used in the future or exchanged with other players outside of battle.
Also, at the beginning of the game, players receive emblems unique to their save file. Getting together with other players and combining emblems can potentially unlock rare items.
All of these multiplayer functions require each player to have a copy of the game. Boktai will be released tomorrow in Japan.
