Who is Lucario?
Quick Stats
Lucario (ルカリオ)
The Emanation Pokemon

Stats (X and Y)
Attack
110
Defense
70
Special Attack
115
Special Defense
70
Speed
90
Catch Rate (Riolu) Calculator
75
Still under construction!
Origin
Lucario (or in Japanese, ルカリオ) was "born" after the 3rd generation of Pokemon, and made his debut in the movie Lucario and the Mystery of Mew. Afterwards, he became the 448th Pokemon from the Sinnoh reigon.
Prominence
Since his début, he has been in every Pokemon video game. He's also been in featured in the anime, under "ownership" of Maylene. In the manga, he's found under Riley. He's also been featured in Super Smash Bros. Brawl, where he is a playable character.
About Lucario
The 448'th Pokemon in the National Pokedex, Lucario is a Fighting/Steel Pokemon, with high Attack, Special Attack, and Speed. He is suprisingly quite short (3'11"), but this makes him very agile. He's a blue, black, and yellowish jackal-like Pokemon, and he has the unique ability to read Aura through the appendages on the back of his head.
Aura
Lucario's specialty is his ability to read Aura, ore the life force inside every living being. Auras are unique to every living being, and emotions can also affect one's Aura. Aura itself is just energy, making it a very versatile tool. It can be used to convey speech, emotions, find hidden people/Pokemon, but its most important use is probably for combat. Analogous to today, it would probably be close to a heat-seeking missile. When Lucario fires an Aura Sphere, it never* misses.
*Somehow it can't hit flying, diving, or undergound targets. Oh well, gotta make the game balanced somehow...
Anyways, read more about him here!