Chopper was originally a reindeer until he ate a fruit that turned him into a human. The transformation was somewhat incomplete since he retains his reindeer appearance even in human form. He is capable to switch between seven different forms of appearance none of which are neither human nor reindeer.
Chopper tends to swear when he is complimented, unable to deal with the situation. It stands to reason that he is still highly affected by approval of any sort.


http://www.youtube.com/watch?v=7F8Si58Fv1g

As a world class cook Sanji knows to never use his hands in a fight to not spoil them for the art of making food. Sanji is most famous for his love for women. Sanji joined Luffy after being kicked out by his former employee for creating damages in his restaurant.

http://www.youtube.com/watch?v=8gS3mnU_vHw

Sanji is along with Luffy and Zorry part of the Monster Trio, other crew members recognize for their ridiculous inhuman strength.

After eating a misterious fruit he becomes immortal, watches his friends die and his own body decay.

http://www.youtube.com/watch?v=CftKJq5ca-Q

Vivi is next ruler in line of a desert country called Alabastia. She joined Luffy to aquire the help of his crew to combat a civil war in her country. After Luffy defeated the leader of the people and secured the artistocracy, her loyalty to her captain has been wavering and she has chosen to remain in her country instead of fullfilling her responsibilities towards the pirate crew. So far Luffy has refrained from beating her up and dragging her back on the ship like he has done with other crew members.
Vivi served together with and under Robin prior to joining up with the Straw Hats.

https://www.youtube.com/watch?v=TGBInH95DeM

Aka Robin Nico Aka Miss All Sunday.

Robin spent her early youth looking for ways to construct WMDs together with some renegade scientists. When the world government found out, it managed to avoid the worst. However since Robin was still a minor she wasn't incarcerated. She started to seek out the worlds worst criminals for support in her research, betrayed them one after another and left a wake of destruction behind her.
She was most prominent as the right hand of Sir Crocodile, a rogue turned vassal of the world government who built an underground army and secretly sought to overthrow the country of Alabastia.
After Crocodile had no more information to offer to her she finally joined the Strawhats upon personal request rather than like everyone else being invited.
Her research has since made huge leaps forward.

After her involvement in the destruction of the government of Dressrosa as part of the Strawhat Crew her bounty was raised to 130 million berry, making hers the highest bounty after the monster trio consisting of Luffy, Zorro and Sanji.

https://www.youtube.com/watch?v=QhXoLhl-C6I

Is a human made entirely of rubber. He is immune to blunt attacks, bullets and generally incredibly resilient towards damage. He doesn't take injuries from heights or crushing into objects which allows him to use his arms as some sort of slingshot as travel method on difficult terrain.
Luffy has spent the first 2 years of his existance in his mothers womb, resulting in a form of severe dementia. Of course his mother didn't survive the birth.
His father is a known internationally wanted terrorist so he grew up with his grandfather, creating a trauma which in turn resulted in his desire to become a pirate. Important coinfluence was certainly an older man called Shanks, who was a male role model for Luffy and filled the present void as a father figure. Shanks helped Luffy along the way to make poor life decisions having poor charakteristical traits himself and being a pirate.

http://www.youtube.com/watch?v=9LEAQay-HdY

Cutty Flam aka Franky was a common criminal in Water 7, an island that mainly lives off of offering ship maintenance and repair. As such Franky is also a ship builder although instead of following his profression he decided to start a circus and to make money through deception and ripping people off.
The people of Water 7 got so annoyed with him that they begged Luffy to take him away. Luffy therefore ordered his crew to steal the mans underwear and only offered to return it upon joining his crew.
Cutty Flam only possessing one pair of underwear had no choice but to agree.


Obnoxious pervert or perfect father figure? You decide.

https://www.youtube.com/watch?v=bfXwegXy2nE

Sogeking aka Usopp is the worlds best sniper and shadow captain of the Strawhats. Nobody really knows that he is but occasionally you can see through the facade he puts on and catch a glimpse where he admits to being the captain. Usopp commands a 10000 men army and is known to be never terrified of anything. He likes to fool his opponents by pretending that he is.
Although the marine has not managed to look through his sharade yet and to realize his true power he still holds a formidable supernova level bounty of 135.000.000 berry. However, the ruler of Dressrosa, former world noble Don Flamingo recently set a private bounty of 500.000.000 berry on his head, a bounty of such abysmal levels is only held by a handfull of people in the world.

https://www.youtube.com/watch?v=OxZDxxD9yw0

Firefist Ace is a friend of the Mugiwara 11 and Luffys older brother. He consists of pure fire and can turn everything around him into fire. As a result he is pretty much involunarable since it's hard to kill fire with a stick. Other elemental damage can probably harm him tho.
Firefist Ace is the second in command under the pirate Whitebeard, an old pirate past his best days who constantly injects booze in his veins and demands of his subordinates to call him "father".
Ace last bounty was 550.000.000 berry.

http://www.youtube.com/watch?v=YdrbNWqY2ww.

This elite pirate ended his outlaw career to join the world government.
He can use his hands to generate a several meters wide quantum field within which he can teleport and switch out objects. An ability which gave him the loving nickname 'the Surgeon of Death'.

Law eventually got tired of the life by the rules and decided to create an alliance with Luffy to go after his former employer, another reformed pirate.

https://www.youtube.com/watch?v=uUbq2t3ns_A

Possibly the best swordsman in the world.

https://www.youtube.com/watch?v=UZq6TgUzKmA