Pokémon Trainer (sometimes referred to as Red) is a red-clothed, 11 year old who is the playable character of the original Pokémon games, Pokémon Red and Blue, their remakes, Pokémon FireRed and LeafGreen, and Super Smash Bros. Brawl, though it's his Pokémon that do the actual fighting.
So far, Pokémon Trainer has appeared in twelve Pokémon games in total. In Pokémon Red and Blue (Red and Green in Japan) and Pokémon Yellow: Special Pikachu Edition, he appeared as the playable character, whereas in the remakes of Red and Blue, he appeared as one of the playable characters along with Leaf. In Pokémon Gold and Silver, their remakes, and Pokémon Crystal, he appeared under the name of Red, who resides at the top of Mt. Silver, and is supposed to be the 'final boss'. His most recent appearance is in Black 2 and White 2, where he appears (again under the name of Red) as one of the opponents in the Pokémon World Tournament. He also appeared in Super Smash Bros. Brawl as Pokémon Trainer, in which he sent out and retrieved the Kanto starter Pokémon in different evolutionary stages.
In M.U.G.E.N, Pokemon Trainer has Ivysaur, Squirtle and Charizard to battle for him just like in Super Smash Bros. Brawl, and was created by MGSSJ2. Although this character acts pretty much like it does in Super Smash Bros. Brawl, it has a huge error in its coding, so when controlled by the AI, he cannot switch Pokémon, so it'll always be stuck with Ivysaur, even when defeated (all three have to be defeated to win).
Like his Brawl counterpart, Pokémon Trainer does not attack himself, but instead uses his three Pokémon (Ivysaur, Squirtle and Charizard) to do so instead. All three Pokémon can be switched between freely, though once a Pokémon has been KO'd, it cannot be switched in until the next round. It is possible to exploit a error in the character that makes the current Pokémon vanish from the screen, by pressing while holding . The player then needs to mash the buttons to reappear on the screen as Ivysaur (this won't work if Ivysaur has fainted).
It should be noted that because the Trainer is the root character, the A.I. opponent may occasionally try to attack the Trainer as opposed to the Pokémon.