Party member

Party members are characters who fight alongside you in combat. Some Party members are consistent and fight alongside you throughout most of the game, some only fight with you in certain worlds, and some only helps out as assisting party members during special events.

Kingdom Hearts Birth by Sleep
Kingdom Hearts Birth by Sleep does not feature set party members and allows only one character for most of the game, granting party members only in a handful of battles. Instead of party members, the game focuses on the Dimension Link system, a successor to the Summon system.

Kingdom Hearts
In certain worlds, a character from that area will be available to fight, and can replace Donald or Goofy in the party. Kingdom Hearts has six such worlds with party members.

Kingdom Hearts: Chain of Memories
In Castle Oblivion, Sora and Riku's party members can only act in battle by appearing as friend cards; this goes for not only the world specific allies who are derived from Sora's memories, but also for Donald, Goofy, Pluto, and Mickey.

Sora has been given cards in Castle Oblivion and is sent to worlds from his own memories and his past party members from Kingdom Hearts from the Disney worlds he had visited have become memory cards.

Kingdom Hearts 358/2 Days
Kingdom Hearts 358/2 Days has several worlds that Saïx sends Roxas to on missions, often along with other members of the Organization.

Kingdom Hearts II
Kingdom Hearts II increasees the number of worlds with party characters to nine, including one in almost every Disney-based world. Three characters from Kingdom Hearts returned for battle: Aladdin, Jack Skellington, and Beast, although Beast was moved from Hollow Bastion to his own world. Ariel reappeared in Atlantica, but no longer as a party character, Peter Pan became a Summon, and Simba, formerly a Summon, became a party member in his home world of the Pride Lands. The game added a party member, Jack Sparrow, with the same first name as another, Jack Skellington; Sora differentiates between the two by referring to Jack Sparrow as Captain. To make the world-based party members more powerful, special attacks were developed for each character that Sora could call on in the heat of battle. Kingdom Hearts II also included the first Final Fantasy party character, Auron, and allowed Sora to fight alongside his best friend, Riku.

Kingdom Hearts coded and Kingdom Hearts Re:coded
The worlds in Kingdom Hearts coded are drawn from those in Kingdom Hearts, though the party members and layout are different as there are only two worlds where Data-Sora gets party members and both worlds feature different gameplay than usual. In Olympus Coliseum, the player controls the party members the same way Data-Sora is controlled but 'Game Over' only occurs when Data-Sora dies. In Hollow Bastion I, Data-Sora can't attack and instead orders what Donald and Goofy does. A special ability, Ask for Help, allows Data-Sora to cycle Donald and Goofy's focus between following Data-Sora, attacking enemies and destroying blox. Data-Sora's own deck commands are replaced with new commands that Donald or Goofy uses, when the player uses the command. Donald and Goofy both have their own HP and if their HP is reduced to 0, they can only be revived by Data-Sora going to them and using Rescue.

Kingdom Hearts 3D: Dream Drop Distance
Kingdom Hearts 3D: Dream Drop Distance does not feature world party members, as they are replaced by Dream Eater Spirits as your party members.