Name: Sazh Katzroy
Japanese name: サッズ・カッツロイ
Occupation: Airship pilot
Height: 6'2" (1.89m) [includes afro]
Weapon: Two pistols
Seiyuu: Masashi Ebara
Voice actor: Reno Wilson
Sazh is an airship pilot. Before the death of his wife three years prior to the game's start, Sazh was all about his job, but after losing her he focused more on being a father. He changed from a long-distance route to a local route, all to spend more time with his son, Dajh, and the trip to Euride Gorge to see a fal'Cie was purely to make Dajh happy. (This is all detailed in the prequel novella.)
However, when Sazh stops to buy a chocobo chick for Dajh, the boy wanders off into the energy plant, where the fal'Cie is. At that time, two Pulse l'Cie happen to be in the plant, and their presence causes the fal'Cie to brand Dajh as a l'Cie. Dajh is taken by the Sanctum for testing, but there's nothing they can do to change him back.
With nothing left to lose, Sazh decides to seek out the Pulse fal'Cie in Bodhum in hopes of saving Dajh, and by chance meets someone with the same idea: Lightning.
Sazh serves as the game's comic relief, but also has some of the darkest parts of the story. I personally feel that Sazh's story is one of the single best parts of FFXIII, and that his character development and relationships with the other characters is absolutely spectacular. While I'm less pleased with what happened with him in the sequels, everything in FFXIII with him is just fantastic to me.
Sazh's default roles are Commando, Ravager, and Synergist. He's by far the game's best Synergist, learning Bravery, Faith, and Haste long before anyone else, but the rest of his Crystarium is variable in use. I feel like Sazh is a character you can only use well if you know exactly what you want to do with him, and unfortunately I've never had a lot of luck myself. That's not to say he isn't useful, just that I haven't gotten the hang of it myself. You can find his full Crystarium here.
His Eidolon is Brynhildr, which I don't find to be useful at all.