Monkey D. Luffy is the seventeen-year-old captain of the ship The Going Merry, and of his mis-matched crew. He dreams of finding the legendary treasure One Piece and becoming the King of Pirates, despite the fact that when he was a young boy, he ate one of the Devil Fruit; a rare fruit that gives the one who eats it special abilities, but only at a price: once he ate it, Luffy discovered that he would never be able to swim again. This hasn't stopped him, however; he uses the abilities the Gum-Gum fruit gave him, the ability to stretch his body like rubber, to further his pirate career. Perpetually cheerful, outgoing, and a bit dense in most areas (but appearing to be a strategical genius), Luffy is an incredibly likeable, honest, and trustworthy person despite his occupation.
Also when he was young, he idolized a pirate captain named Captain "Red-Haired" Shanks (who has got to be one of the most kickass characters in the series). He cut himself under the eye (hence the scar) to prove to Shanks that he was tough enough to be a pirate, but Shanks didn't buy it, and told him that cutting himself was a stupid thing to do. Later, when a gang of bandits insulted Shanks while the captain wasn't there, Luffy attacked them in Shanks' defense — and was subsequently thrown into the ocean. This being after he'd eaten the Gum-Gum fruit, he obviously couldn't swim. A massive sea monster attacked, but Shanks grabbed Luffy before he could drown and more or less told the monster to get lost, which it did — probably because it was terrified that it had bitten off Shanks' arm while the man defended Luffy and the captain hadn't so much as flinched.
Luffy idolizes Shanks and strives to be just like him — and vows to acquire the One Piece, the executed "Pirate King" Gold Roger's legendary treasure.
Roronoa Zoro is nineteen years old, and is already known as the infamous Demonic Beast. He is — or rather, was — a pirate hunter whose name made most people rather... jittery, to say the least. Some people believe that he is really a demon in human form. Zoro practices 'Santoryu', or three-sword style. With a sword in each hand and one in his mouth, joined with his incredible skill, Zoro is a frightening person when he's in battle mode. When he first met Luffy, he was imprisoned by Captain Axe-Hand Morgan, a ruthless and vicious Navy officer, for killing the dog of Morgan's son, Helmeppo. Zoro was told that if he could last a month without food or water, he would be set free — this was a lie, for he was to be executed in three days' time. Luffy, upon finding this out and discovering that Zoro had killed the dog to save a little girl, was determined to make Zoro part of his crew. Retrieving Zoro's three swords and releasing him, the two kicked some serious ass and defeated Captain Morgan, freeing the town from his tyrannous rule.
When Zoro was young, he had a rival — a young girl named Kuina, the daughter of the man he trained under, whom he challenged 2000 times in a row — and lost to, 2000 times in a row. He simply couldn't defeat her, despite the fact that he used two bamboo training swords and she only used one. On their 2001st duel, Zoro challenged her to use real swords — she accepted, and they dueled. Once more, Zoro lost. Frustrated with his loss, he was shocked to find that she was upset — she sat down to cry on the steps on her father's dojo. She explained to him that while men get stronger with age, women get weaker and thus, she could never be the greatest swordsman or -woman in the world. Her father had told her this to crush her dreams. Zoro more or less told her she was being a moron, and they made a vow together to both strive to be the greatest swordsmen in the world, and they'd eventually fight for the position.
Unfortunately, the next day Zoro was told that Kuina was dead — she'd fallen down the stairs. He requested to have her sword, and vowed to keep their promise for the both of them and become the greatest swordsman in the world.
Despite looking like a badass — and actually being one when in a fight — Zoro is actually quite a kind person. He and Luffy appear to have some sort of freaky telepathic link, for when in a fight especially, they only have to look at each other or say a few words and they immediately know what the other wants them to do. Outside of a battle, Zoro tends to understand Luffy's eccentric ways the best out of all the crew.
And just for the hell of it...
...a conversation with my friend. I am Fire Tears.
Chevira: Their ages. How the fuck is Luffy a captain at SEVENTEEN?
Fire Tears: There's no age limit, per se. They're PIRATES, it's not like they're going to go, "Honey, you're underage. Toddle back off to your bubble-bath toys until you're eighteen."
Chevira: I know that—but how could he get a crew loyal to him, a ship of his own, and whatnot? By seventeen?
Fire Tears: He... just started out. When he turned seventeen he left his hometown in this pathetic little rowboat and found Zoro the Pirate Hunter. And things progressed from there. :D
Chevira: ....*sweatdrop* And as in all things anime-y, it all just worked out.