Aura Sphere is Lucario's neutral B. Like Mewtwo's Shadow Ball from SSBM, it can be charged, the charging animation has a hitbox that does a small amount of damage, and it can be held in the charging animation after it is fully charged. Unlike Mewtwo's Shadow Ball, it moves in a mostly straight horizontal line when fired, and at a faster speed. Its knockback does not seem to be quite as powerful, however.
Aura Sphere can be considered Lucario's signature attack, learn able by only a select few other Pokémon (Like Mewtwo) and is featured prominently in the movie Lucario and the Mystery of Mew as the prime example of Lucario's ability to manipulate aura.