Toon Zelda isn't a character, unless you are talking about toon link. (a more common mistake than most people believe)
Toon Link can be unlocked by:
1) Playing 400 matches in Brawl, and then beat Toon Link in a fight.
2) Beating Classic Mode on any difficulty (with Link, Zelda, or even Ganondorf), and then beat Toon Link in a fight. Can only be done after the Subspace Emissary is completed.
3) Complete the Subspace Emissary, then return to Stage 9 (The Forest). On the first portion of the level after the first mini-battle with a few Primids, a new door appears. Go in and beat Toon Link in a fight, then finish the stage.
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Wind Waker and Phatom Hourglass
The only way to get Toon link is to finish Subspace Emissary first. After that, go to the map that link took the master sword. Toon Link is hidden in that map, if you find him, you have to defeat him, after he is defeated, you get Toon Link. I think the easier way than that is to finish any standard level in any one of the characters.(Link, Zelda, Ganondolf) After defeating master hand, Toonlink will show up for battle.
My brothers and I unlocked every character by doing enough multiplayer games. That's the only way I know.
Toon link can be unlocked after beating tabuu at the great maze then going back to the forest level and when you get past the first mini brawl you will soon find a red door that never was there before go in and toon link will appear defeat him and he will join your team save and quit and you will unlock him.
You have to do a certain mission to unlock the head, then you beat the cog bosses which are like lvl 60 so you should bring lots of friends, but only sometimes do you unlock the parts of the body of the cog.