Pichu's main gimmick is that for every electric attack it uses, it damages itself. Combined with it being lighter than the already-light Pikachu, it is very easy to KO. Unique to Super Smash Flash 2 is that, after Pichu reaches 50% damage, it will enter a fatigued state called Discharge, where its special moves become stronger or gain new traits: Electro Ballparalyzes the opponent for a much longer time, Thunder is stronger and propels Pichu upwards, Agility produces an electric vortex after each dash and Skull Bash goes significantly farther, but cannot be charged.
Pichu is even smaller and faster than Pikachu, as said previously. He can dash cancel to move even faster than Sonic, giving him overwhelmingly good approach options.
However, with his small hurtbox also comes a very severe problem with range. Pichu must get way too close to the opponent in order to profit off his quick attacks. Combined with the fact that most of his moves are reverse disjointed, Pichu ends up having lots of good approach options, but still having issues with winning the neutral game. Pichu is also the second lightest character in the game, weighting 73 units, while Jigglypuff, the lightest, weights 72.
Pichu spawns a lightning bolt above him. If it connects with Pichu, an extremely strong hit will occur and Pichu will get sent upwards in tumble state, allowing for infinite recovery in some situations.
Pichu's splash screen, from his introduction trailer.
Pichu is the first character that was originally implemented as a Poké Ball character to become a playable character.
Out of the three characters to ascend from being item-only characters to becoming playable (the other two being PAC-MAN and Bandana Dee), Pichu is the only one to do so in the same version that it was introduced as a non-playable character.
However, as confirmed by the devs, both the playable Pichu and the Poké Ball Pichu can appear at the same time.
Pichu, like Pikachu, is confirmed to be a male Pokémon.
Despite its gimmick, Pichu only has two moves that actually damage itself in the actual Pokémon games.
Pichu is the only character in the game that has four taunts, referencing how he also had one more taunt than the other fighters in Super Smash Bros. Melee.
Pichu, Ness, and Zero Suit Samus are also the only characters that can self-heal from damage. In Pichu's case, via crouching for some time.
Having to deal 172% damage on Pikachu in order to meet Pichu is a reference to the latter's National Pokédex number.
Pichu alongside Naruto, Lloyd Jigglypuff, and Sandbag are the only characters that don't clap for the winner of a match.