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Battle Card was an item coming from Kingdom Hearts: Chain of Memories which would cause multiple effects on the summoner or, in most of the cases, summon a random non-playable character to aid the player who summoned it in Super Smash Flash 2. Those Battle Cards would have worked identical to the Assist Trophy item, the only difference being that a Kingdom Hearts character would appear. Battle Cards would come in Red, Blue, Green, and Black colours, but all would work the same. The DOJO!! has stated on April 14th, 2012 that Battle Cards will no longer be a part of SSF2.
Cards first appeared in the game Kingdom Hearts: Chain of Memories where they were used by Sora and Riku to fight and proceed through Castle Oblivion. There are two types of Cards: Battle Cards and Map Cards. Battle Cards, in turn, are divided in Attack Cards, which contain one strike with a Keyblade (they are colored red); Magic Cards, which let Sora use a spell or use a summon (They are colored blue); Item Cards, which restore attack and magic cards after use (they are colored green); Friend Cards, which summon one of Sora's party members once retrieved and used (most likely the color of card that will summon characters in SSF2, they are colored green too); and Enemy Cards (enable a status affect, which can change Card Value, increase speed, and make Sora invulnerable to certain elements. They are colored black). There are other types of Cards but are not in the Battle Cards line.
List of Battle Card characters
Genie (Aladdin): Would send a puff of magic at each opponent.
A rumor among McLeodGaming community was that a summoning item in the form of card would appear as an item. This led to the belief of Yu-Gi-Oh! cards as an item in SSF2, which spread a discussion all over the SSF2 community. Nothing was official until the confirmation of Battle Cards.
Since the Battle Cards are no longer a part of SSF2, it is possible that the characters that were planned for use of the Battle Cards will still be used as Assist Trophies. However, because they are all a part of the same franchise, not every character would be included.