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.
In Pokémon Red and Blue and Pokémon Yellow: Special Pikachu Edition, Red is the sole playable character, who travels the Kanto region catching Pokémon and collecting the 8 gym badges necessary to challenge the Elite Four at the Indigo Plateau and become the Champion of the Kanto region. During his travels, he repeatedly encounters Blue, his childhood rival, who manages to become the Champion before he can, while simultaneously encountering and defeating Team Rocket and their boss, Giovanni. In the remakes Pokémon FireRed and LeafGreen, his role is the same, except he is now one of two possible playable characters, the other being Leaf. In Pokémon Gold and Silver, Pokémon Crystal, and Pokémon HeartGold and SoulSilver, he appears at the top of Mt. Silver, acting as the true final boss of the games. 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 K.O.'d, it cannot be switched in until the next round. It is possible to exploit an 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.