A Final Form is a type of Final Smash 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 become more powerful than the regular ones, 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 take less damage 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 do 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.
It was originally considered by developers that during the transformation, 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 has transformed. However, it is no longer confirmed to be present in the game.