Monica Geller is a little bit crazy, but definitely seems to be the leader of the group of Friends.  She's Ross's sister and high school best friends with Rachel and kind of ties to the group together as a mother-like character.

Monica is known for being a little OCD and for being overweight in high school.  Now?  She's a bit of a chef but holds various jobs during the season.

At the end, Monica ends up marrying Chandler in season seven.

Monica Geller Quotes

Rachel: Julie! Julie, isn't that great? I mean, isn't that just kick-you-in-the-crotch, spit-on-your-neck fantastic?
Monica: (To Julie) It's an expression.

Monica: What's "PLEH?"
Joey: That's help spelled backwards so that the helicopters can read it from the air!
Monica: Ah...what's doofus spelled backwards?

Friends Quotes

Rachel: Daddy! Daddy listen to me! It's like all my life everyone's told me, "You're a shoe! You're a shoe! You're a shoe!" Well, what if I don't want to be a shoe? What if I wanna be a purse or a hat? No I don't want you to buy me a hat, I'm saying I am a hat. It's a metaphor Daddy!
Ross: You can see where he'd have trouble.

Rachel: Julie! Julie, isn't that great? I mean, isn't that just kick-you-in-the-crotch, spit-on-your-neck fantastic?
Monica: (To Julie) It's an expression.