A Pokémon Trainer (ポケモントレーナー) is a human in the fictional world of Pokémon whose hobby, pastime, or profession is the collecting, caretaking, and competitive battling of any of the eponymous Pokémon creatures. The humans being the commanders of their custom-trained Pokémon in battles is as important as the abilities of the Pokémon themselves in all forms of Pokémon media.
From the very outset of the Pokémon franchise, the world of Pokémon had been established as an Earth-like world populated by humans and, notably, an infinitely diverse biomass of magical creatures called Pokémon (short for Pocket Monsters). In this world, humans are referred to as Pokémon trainers if they collect, take care of, and train Pokémon for use in competitive matches with those of other trainers called Pokémon battles, officially sponsored and otherwise. Other Pokémon-related hobbies and professions exist in the world, such as Pokémon breeding, and a wholly different activity is undertaken by Pokémon coordinators who groom their Pokémon for use in Pokémon contests more closely resembling dog shows, but by far the path of the Pokémon trainer is the "primary" sporting activity in the franchise. This is the main activity/quest that the "main" Pokémon RPGs feature; as such, a player of a Pokémon RPG is considered a Pokémon trainer himself or herself.
Any Pokémon RPG casts the player as a young Pokémon trainer ready to set out on a Pokémon journey across that game's region of the Pokémon world, on a quest to capture many species of Pokémon and, from them, train a team of Pokémon proficient enough in the sport of Pokémon battling that they may carry the trainer to victory against that region's Pokémon League. The trainer's traditional tool for capturing Pokémon is the Poké Ball, which is sold at Poké Marts in the game's various cities and comes in many distinctive varieties, and a new trainer receives a piece of high-tech equipment called the Pokédex, which functions as an encyclopedia for the Pokémon in that region which the trainer must fill up, among other adventuring gear.
Throughout each journey, the trainer will battle hundreds of other trainers as well as encounter countless Pokémon in the wild, and the trainer will be involved in subplots involving thwarting the schemes of criminal organizations of trainers whose name traditionally starts with "Team", such as the first and second generation's Team Rocket, the third generation's Team Magma and Team Aqua (among other Teams in GameCube Pokémon installments), the fourth generation's Team Galactic, and the fifth generation's Team Plasma. While the Pokémon Trainers have been depicted as boys in every installment, from the Crystal version of Pokémon GS onwards, the player has had the option to choose the gender of the trainer; from then on, each new set of installment has introduced a new pair of standard Pokémon trainer designs, such as a pair each for Pokémon Advance, the GBA remake of the original (featuring a new redesign of the playable trainer from the original Red and Blue versions often called "Trainer Red"), and Pokémon Diamond/Pearl. A small concession made by the franchise to the changing times is the gradual change from the "rivals to the death" image of the first series (Yellow, Red and Blue), to the now very casual friendly talk between the protagonist and their neighbor of Pearl/Diamond. In the Generation 3 games; Ruby, Sapphire, and Emerald, the rival took the gender opposite to the one chosen by the player.
The profession of the Poké Ball-throwing Pokémon trainer is easily as iconically significant to the overall Pokémon franchise as the Poké Ball itself, and is often the image that the Pokémon franchise in general conjures. Perhaps that is why Super Smash Bros. Brawl, in addition to featuring individual Pokémon such as Pikachu as playable fighters, has now been revealed to feature the generic Pokémon trainer as a playable entity all the same. However, the trainer is not the one that does the fighting; he stays away a safe distance as he "directs" his Pokémon, played by the player, to do the battling, just like the trainer would in normal Pokémon battles.
In Super Smash Bros.
In Super Smash Bros. Melee
During the planning stages of Melee, Sakurai considered including a playable trainer, who summoned the second generation starters, but decided against the idea as there wasn't enough time and resources for such a labor-intensive character. In the final game, trainers otherwise have no appearance in Melee outside of trophies.
In Super Smash Bros. Brawl
The Pokémon Trainer makes an official appearance as a playable entity in Super Smash Bros. Brawl, modeled after the male trainer design seen in the Red and Blue versions, and their remakes in which the design was from the character Red. However, it would be inaccurate to call him a playable "fighter", as selecting him is like selecting a Zelda/Sheik-style character with three switchable forms, which are the three Pokémon the Pokémon Trainer has in his collection: Squirtle, Ivysaur, and Charizard. The trainer visually issues commands in the background of the stage to the Pokémon in the foreground, corresponding with the actions input by the player to the current Pokémon as the real fighter. One, however, might say the player controls the Trainer, who commands the Pokémon.
Pokémon Trainer is ranked 29th on the current tier list in E tier. His tournament results have been surprisingly high due to the efforts of TheReflexWonder, but when he is excluded, he has had barely any tournament success.
In Super Smash Bros. 4
It has been confirmed that mid-match character changes will no longer exist in Super Smash Bros. 4. Since the fundamental concept of Pokémon Trainer was based around switches, along with the fact that one of his Pokémon, Charizard, has been confirmed to return as a solo character, Pokémon Trainer has been essentially disconfirmed to return as a playable character, at least in the same style he had in Brawl.