Which Kingdom Hearts game was first to feature cooperative multiplayer?

Kingdom Hearts 358/2 Days was the first Kingdom Hearts game to feature cooperative multiplayer.

Released for the Nintendo DS in 2009, 358/2 Days follows Roxas during his time in Organization XIII. Its multiplayer mode is separate from the single-player story and is organized around missions. Up to four players can cooperate, each selecting from Organization XIII members with different weapons, abilities, and statistics.

The multiplayer also includes competitive battles, a chat system, and unlockable characters such as Sora, Riku, Mickey Mouse, Donald Duck, and Goofy. Because multiplayer access could not be assumed, the missions were designed to remain playable with computer-controlled partners when other players were unavailable.

This distinction matters because other Kingdom Hearts games also include multiplayer or multiplayer-like features. Birth by Sleep, for example, has multiplayer modes, but it was released later than 358/2 Days. Re:coded's Nintendo DS tag mode registers nearby players as “ghosts,” yet that is not the same as the mission-based cooperative multiplayer introduced by 358/2 Days.