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.