M o n k e y  d . L u f f y
"luffy of the straw hat" / captain, straw pirate fleet
Age: 17
Birthday: May 5
Originally From: Fusha Village
Devil Fruit: GumGum Fruit
Intelligence: relatively low
Personality: sanguine

Hobby: eat (especially meat)
Holds Dear: Shanks' straw hat, friends
Dream: Find One Piece and become the Pirate King

Position: Captain

Person who impressed him: Shanks

First Appearance: first episode 
Needless to say, Luffy is the main character of this series. Monkey is his last name because in the OP universe, when there are full names, it is usually last name first. He seems to be wanting a musician in his fleet. When he was seven years old, he accidentally injected the GumGum Nut (a devil fruit) that was on Shanks' ship, and became a rubber band boy that could not swim. Instead of being afraid of water, 
He uses his stretch ability to become the best pirate ever. He seems a little loose, but is is very focused on his goal of finding the One Piece and becoming the King of Pirates. He also looks forward to the day he can meet Shanks again to return the straw hat as well.
It may also be of note that the scar below his left eye resulted from him stabbing himself to show Shanks he was brave enough to become a pirate.

R o r o n o a  Z o r o
"zoro the pirate hunter" / swordsman, straw pirate fleet
Age: 19
Birthday: November 11

Height: 178 cm

Hobby: sleeping (almost at the wrong time and wrong place!!)

Person who impressed him: Kuina (and Luffy, maybe)
Originally From: ??
Devil Fruit: none
Intelligence: higher end of fleet, but very low on sense of direction
Personality: short-tempered but kind
Holds Dear: pride as a swordsman, his three swords
Dream: become the strongest sword master in the world

First Appearance: first arc
Zoro started out as a bounty hunter that had been captured by Morgan, and was being held captive in the Marine port town that Luffy and Coby first landed for food. He ultimately serves Luffy in the boy's fleet as his right-hand man and a 3-sword swordsman. His goal in life, which he swore to the grave of Kuina, is to become the strongest swordsman in the world. His last name comes from an real-life pirate, Francois L'ollonois; but the folks who romanized the names decided that it was too hard to spell or something.

U s o p p
"captain usopp" / marksman, straw pirate fleet 
Age: 17
Birthday: April 1
Originally From: Syrup Village
Devil Fruit: none
Intelligence: mediocre

Hobby: lie
Personality: easily scared, master of bluffs
Holds Dear: xx
Dream: xx
First Appearance: Kuro arc

Person who impressed him: Kaya

Position: shooter, but sometimes he do some extra job like repaired machine, etc

A 17-year-old who was known as the boy who cried wolf (or rather, the boy who cried pirate) in his hometown. He has pride that his father was a pirate who sails with Shanks, and created a mock-pirate group of his own in his town. His is a fine sniper with many different kinds of "ammunition" on his slingshot, which he carries in his waistpouch that he always carries.

S a n j i 
chef, straw pirate fleet
Age: 19
Birthday: March 2

Height: 177cm

Special Ability: cooking
Originally From: ? - Orbit - Baratie
Devil Fruit: none
Intelligence: relatively low (but smarter than Luffy)
Personality: girl-crazy and short-tempered
Holds Dear: women

Person who impressed him: Zhef
Dream: to find All Blue
Position: chef

First Appearance: Baratie arc
When Sanji met Luffy, he was the chief assistant to the owner/head chef (Zeff) aboard the Baratie. Looks like 25 but actually is 19. Always smoking a cigarette. Never shows his left eye. Eyebrow is extremely curly (Zoro later calls him "wonderful eyebrow" during one of their arguments for that reason.) Never uses hands to attack,but he doesn't need hands because his kicks can turn rocks into sand! His dream is to find All Blue, the legendary area somewhere along Grand Line where fish from all four seas live. Very short-tempered against men, but a sucker for beautiful women (even to their enemy!!). He initially doesn't want to leave the Baratie because he doesn't feel he has repaid Zeff enough... but Zeff asks Luffy to take Sanji along to the Grand Line, and he ends up on the boat with much tears.

N a m i
navigator/thief, straw pirate fleet
Age: 18
Birthday: July 3

Height: 169 cm (B86, W57, H86) 
Originally From: Cocoyashi Village
Devil Fruit: none
Intelligence: high
Personality: sly, somewhat selfish
Holds Dear: money, orange, tangerine

Person who impressed her: Bellemere
Dream: get a lot of money

Position: Navigator

First Appearance: Buggy arc
The second person that Luffy decides to invite into his fleet is a curvaceous (34-23-34 according to Sanji) 18-yr-old girl who claims to be a thief who steals only from pirates. She is a master of navigation, and the maps she draws of the seas are perfect, and is deadly with her wand (that she usually hides in 3 pieces up her skirt). She justifies her stealing by saying that she needs to collect 100 million bellies and buy a certain town. She hates pirates and loves money (and tangerines)—because pirates took someone dear away from her, and tangerines remind her of that someone dear.

Tony Tony Chopper

age: 15

Birthday: 24 december

Special Ability: talking with animal

Intelligence: High

Personality: shy, easy to scare

Position: Doctor

Dream: to become the greatest doctor