Unlockable

Info
There are unlockable things in Super Smash Flash. There are below:

Luigi
Clear Adventure on at least normal with Mario.

Jigglypuff
Clear Adventure on any difficulty with any character.

Cloud
Complete 100-man melee with any character.

Crono
Clear 3-minute Melee.

Naruto
Clear Adventure with the original 13 characters.

Shadow
Clear Adventure with Sonic, Tails, and Knuckles

Super Sonic
Clear Classic with Sonic on Very Hard Difficulty without getting Game Over. (Known glitch that you may be on any difficulty and it will still work)

Zero
Clear Adventure with Megaman on at least Normal Difficulty without getting a Game Over.

Young Link
Clear Classic with Link on any difficulty.

Inuyasha
Clear Adventure without losing any lives.

Mr. Game and Watch
Clear Target Test with every character but him.

Mr. Incredible
Clear Classic Mode with the original 13 characters.

Mewtwo
Clear Adventure with Pikachu and Jigglypuff.

Lloyd
Clear Adventure with any character after having one hour of total play time.

Blue
Clear Adventure with Blade on at least Normal difficulty.

Final Destination
Unlock every character

Battlefield
Clear All-Star Mode with any character

Modes
All-Star is available after you have every character.

Trivia

 * The best way to unlock Final Destination and All-Star Mode is to unlock Mr. Game and Watch.