The newest playable character in Super Smash Bros. Brawl is Ash...or whatever you named him in the Pokemon games.
The newest character in SSB Brawl is... Pokemon Trainer! This nameless trainer hangs around in the background, controlling one of three Pokemon who actually do the fighting for him. Pokemon Trainer always has access to Squirtle, Ivysaur, and Charizard, who share the same damage percentage. You rotate through them by using Pokemon Trainer's "Down" special attack.
Note that each of these three Pokemon is at a different stage of evolution, perhaps indicating that Squirtle is the weakest and Charizard the strongest. However, Pokemon Trainer is balanced so that you can't just pound away with the strongest one all the time. The three Pokemon have a stamina characteristic (apparently conveyed through animations) which goes down when a single Pokemon is used too much or when Pokemon Trainer rotates through them too frequently. Loss of stamina seems to affect the Pokemon's effectiveness in battle.
It is highly likely that each of the Trainer's three Pokemon will have different special attacks, making this character a more complex version of the Zelda/Sheik mechanic from Super Smash Bros. Melee.