Red (known as Pokémon Trainer in Super Smash Bros. Brawl) is a red-clothed, 11 year old Pokémon Trainer from the Kanto region, 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, Red has appeared in 14 Pokémon games in total. In Pokémon Red and Blue (Red and Green in Japan) and Pokémon Yellow: Special Pikachu Edition, he appeared as the sole 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 appears at the top of Mt. Silver, acting as the games' true 'final boss'. He appears again in Black 2 and White 2, where he appears as one of the opponents in the Pokémon World Tournament. His latest appearance in Sun and Moon has him as a young adult visiting the Alola region along with his longtime rival Blue, both of which are opponents at the Battle Tree. He also appears in Super Smash Bros. Brawl under the name Pokémon Trainer, in which he sent out and retrieved the Kanto starter Pokémon in different evolutionary stages.
In M.U.G.E.N, Red has been made by MGSSJ2 under the name of Pokemon Trainer; being based on his appearance in Super Smash Bros. Brawl, he has Ivysaur, Squirtle and Charizard to battle for him and can switch between them, though the A.I. is unable to do this, so it'll always be stuck with Ivysaur, even when defeated (all three have to be defeated to win).
Pokemon Trainer functions similarly to his Super Smash Bros. Brawl counterpart in that he himself does not attack, but instead uses his three Pokémon (Ivysaur, Squirtle and Charizard) to do so; 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 (Z) 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 Pokemon Trainer is the root character, the A.I. opponent may occasionally try to attack him as opposed to the Pokémon.