A Final Form is a type of Final Smash in Super Smash Flash 2 where a character transforms into a stronger variation of the character with a full moveset and can be controlled as a normal character.
The Final Form's moves are more powerful than the regular ones, and some are even infused with a certain element of the character transformed. They also gain completely different attacks and special moves to improve them. These characters typically take less damage and knockback and have knockback resistance, which makes them hard to KO. For some Final Forms, like Kyūbi Naruto or Super Saiyan Goku, the transformation can also deal damage to characters near them.
In a normal battle, the transformation time is measured with a small yellow bar that appears above the character's damage percentage counter. Once it is empty, the character will return to his normal form.
Originally, Sonic's Final Smash allowed him to transform into Super Sonic, a Final Form with much more speed. However, it was reworked to a standard transformation Final Smash in Beta.
It was originally considered by developers that during the transformation into a Final Form, a special tune that matches the Final Form would be heard; for example, Super Sonic's theme from Sonic the Hedgehog 2 would have played once Sonic had transformed. However, it is no longer confirmed to be present in the game.
Initially, a Group game mode called Final Form Brawl was considered for the game, in which Final Forms of characters could be selected like regular characters to fight each other. The Final Forms would last the entirety of the match, and their knockback resistance values would be set back to their normal forms to allow standard gameplay. However, this mode has since been cut.