Pichu is described as small bipedal mouse with an oval-shaped head and large, black, arrow-shaped ears. The pre-evolutionary form of the popular Pikachu, the Pokédex explains that the electricity pouches on its cheeks are still small and not able to store much electricity yet; because of that, it is not yet skilled at storing electricity.
In the game series, as the pre-evolved form of Pikachu, Pichu is less powerful than its evolved counterpart, and can't learn some moves that Pikachu can. However, the only way a player can obtain a Pikachu that knows Volt Tackle is through breeding a Pichu.
Pichu makes a playable appearance in this game, albeit as one of the weakest characters, but faster and slightly stronger than Pikachu. Melee is notably the only game that is true to Pichu's Pokédex description of harming itself with its own electrical energy: whenever Pichu performs an attack that involves electricity, it takes a few percentage of damage. It is also the lightest character in Melee.
A Tiny Mouse Pokémon identifiable by its big ears, Pichu evolves into Pikachu. It's not very skilled at storing up electrical energy and will sometimes discharge it if jarred. Although it's small, it has enough electrical power to give even an adult quite a shock. It's often startled by its own power.
Pichu makes a appearance in Brawl as a collectible trophy. It is obtained by completing All Star mode without using a continue. Pichu and Mewtwo were the only two non-returning Melee characters to have trophies in Brawl, and neither trophy description makes any reference to their appearances in Melee in much the same way that none of the main character trophies in Melee and Brawl make any reference to their appearances in any Smash games.
A Tiny Mouse Pokémon. Because its electric cheek pouches are still small, it cannot store electricity yet. Pichu has been known to flaunt its courage by touching tails with others and setting off sparks. It is easier for Pichu to build up electricity when the humidity is lower. Pichu will evolve into Pikachu.
The wall jump ability used by Pichu in Melee is usable by Pikachu in Brawl. That, along with the fact the blue goggles Pichu wore as an alternate costume in Melee appear for Pikachu in Brawl and Pikachus can only learn Volt Tackle through the Pichu stage, have led players to joke (or believe) that Pikachu in Brawl is the evolved Pichu from Melee.
While partial programming information for most fighters that appeared in Melee but not Brawl has been discovered in Brawl's code, Pichu is one of two characters (alongside Young Link), to not have any programmed data at all.