An expansion character, also known simply as an expansion, is a playable character that is not available in the Super Smash Flash 2 roster as a normal character (those who are not starter or unlockable) but can be downloaded into the game making the roster bigger everyday.) With this concept, many characters that didn't get into the SSF2 roster, may be added, to the content of many people. Once downloaded, the expansion is playable on every game mode in SSF2, bar Flash of Shadows. Besides downloading the character, one also acquires extra trophies for said character by clearing Classic Mode, Adventure Mode and All-Star Mode, it should be noted that the trophy's stand for expansions is gray rather than the normal brown.
There is a strict any and all policy on who can be expansions. If anyone wishes to make his own expansion character, whether it is a fan character, a favorite video game character, or an original character, will have to go to the Super Smash Flash 2 Expansions Forums and show the animated sprites, moveset concept and many other stuff so it could be judged and qualified by the Expansion Acceptation Team (EAT for short). Despite this, it has been confirmed that if online matches are possible, only expansions made officially downloadable by the SSF2 Devs (thereby known as Official Expansions) will be playable. Despite the game's incomplete state, Developers have stated that they are working on some expansions to test coding as of now.
List of Official Expansions
None at the moment