Pichu (ピチュー, Pichū) is a fictional creature from the Pokémon media franchise, in which it is the pre-evolved form of Pikachu. It appears as a playable character in Super Smash Bros. Melee, and as a trophy in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS.
Pichu is described as small bipedal mouse with an oval-shaped head and large, black, arrow-shaped ears. As it is the pre-evolutionary form of Pikachu and a baby Pokémon, the Pokédex explains that the electricity pouches on its cheeks are still small and not able to store much electricity yet. It may discharge power accidentally and spontaneously if amused or startled, and vice versa. Despite its small size, it can zap even adult humans, but it surprises itself whenever it does so. When it plays with other Pichu, they may short out electricity with each other, creating a shower of sparks, startling them and causing them to start crying. It gets better at holding power as it grows older, where it starts playing with other Pichu by touching tails and setting off sparks in apparent mutual tests of courage. It charges itself with electricity more easily on days that either have very dry air or has thunderclouds, in which case the crackling of static electricity can be heard coming off of it.
In the games, Pichu only evolves into Pikachu when it reaches a certain level of friendship with its Trainer. Its Pokédex description claims that it is inexperienced with its ability to handle electric-attacks and occasionally inflicts damage to itself, though this does not actually occur in the Pokémon games. It however, has been demonstrated in both the Pokémon animated TV series and in Super Smash Bros. Melee.
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.
In Super Smash Bros. Melee
As a playable character
Pichu is a playable unlockable character in this game, albeit as a percieved "joke character", being an intentionally inferior clone of Pikachu with many significant weaknesses. 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'll sustain 1-4% in damage. Along with this, it is also the lightest character in Melee, giving it extremely poor survivability overall. It is ranked 25th on the Melee tier list in F tier, at the second lowest spot.
During the planning stages of Melee, Sakurai wanted to include a second generation Pokemon, but he found none of the popular options feasible. When Pichu was considered, Sakurai decided against it, as he thought Pichu was too similar to Pikachu. Thinking back to Pichu however influenced Sakurai's decision to add clones to pad the roster, thus leading to Pichu being added into the game as a Pikachu clone and as a second generation representative.
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.
In Super Smash Bros. Brawl
Pichu is one of the five playable characters in Melee who didn't return as a playable character in Brawl, and is the only one who neither has unfinished data on Brawl's disc nor a new playable character as a clear replacement. Instead, Pichu only appears as a trophy. However, a couple of its unique attributes in Melee (its ability to wall jump and the blue goggles it wore on its blue palette) were given to Pikachu.
As a Trophy
Pichu makes a appearance in Brawl as a collectible trophy. It is obtained by completing All-Star mode without using a continue. The trophy describes Pichu's appearance in the Pokémon games, and makes no reference whatsoever to its appearance in Melee:
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.
Pichu returns as a trophy in Super Smash Bros. for Nintendo 3DS.
Trophy in Super Smash Bros. for Nintendo 3DS