Meta Knight is a popular character in the Kirby series and is the second of Kirby's rivals, the first being King Dedede. He follows a code of honor, often providing Kirby with a sword to battle with (only breaking that code when he's not in control of himself), and fights using a golden spiked sword called Galaxia. Meta Knight is also shown to help Kirby sometimes.
In the anime, he's definitely on Kirby's side, helping him fight against the evil Demon Beasts created by the Holy Nightmare Company that Dedede buys from them. He shows some relationships with one of Kirby's friends, Fumu, seeing her as a rare source of intelligence. He's also a Star Warrior, having been one before Kirby and having been one of the few survivors of the war between the Galaxy Soldier Army and the Holy Nightmare Company.
He is also playable in the Super Smash Bros. games, starting with Brawl. In Brawl specifically, he is notorious for being a "god tier" character, which was addressed in the later Smash games.