Monica Geller is the younger sister of Ross, best friend of Rachel and future wife of Chandler.
In her childhood, Monica was extremely overweight, as shown in flashbacks during the course of the show. She shed the pounds after one Thanksgiving, after Chandler (who was Ross' roommate in college at the time) called her fat.
In the first series, Monica was a chef at a restaurant called Iridium. She lived in Greenwich Village in an apartment which she sublet from her grandmother. Initially, she shared the apartment with Phoebe, who moved out after she became concerned that her friendship with Monica would be strained due to Monica's obsessive need to be clean.
When her old friend Rachel arrived at Central Perk after running out on her wedding, Monica allowed Rachel to stay with her.
Monica didn't have many relationships in Series One. She went out with Paul (The Wine Guy) briefly in the pilot, but dumped him after he lied to her to get her to sleep with him. She also went out with Alan, but became disillusioned when her friends seemed to love him more than she did herself. There was also Ethan, but that ended when Monica discovered that he was in fact a senior in high school, not in college as he had originally told her. Also in Series One, it is learned that Monica believed that her parents loved Ross more than her. It also seemed that Judy Geller, Monica's mother, was overly critical of her daughter, but didn't realise it. This became a recurring theme in the series.
In Series Two, Monica was promoted at her restaurant, before quickly being fired for accepting a gift from a food distributor. She remained unemployed for the remainder of the second series.
Monica then met Richard Burke, an old family friend. Even though Richard was 21 years her senior, she quickly fell in love with him. Throughout the second series, Monica and Richard had a strong and happy relationship, but it ended in the series finale when Richard said to Monica that he didn't wish to have anymore children, even though Monica was desperate to have her own family.
At the beginning of Series Three, Monica struggled to cope without Richard, only finding relief when her father told her that Richard was having just as a rough time as she was. The couple very briefly reconciled, after Monica bumped into Richard at a video rental store. But, as they wanted different things, Monica knew that it would be pointless to get into another serious relationship with him. Monica got a job at a 1950s-themed diner, where she had to wear a blonde wig, rollerskates and large fake breasts. It was at this diner where she met millionaire Pete Becker, who continuously asked Monica to go out with him. When she agreed, he took her to Rome for pizza. But try as she might, Monica failed to find any romantic feeling for Pete, until she kissed him for the first time. However, their romance was short-lived after Pete began competing for the Ultimate Fighting Champion title. Being unable to watch Pete get hurt, Monica ended the relationship.
It was in Series Four when Monica began a short-lived catering business with Phoebe. She is also chosen to be a food critic at the Chelsea Reporter. She wrote a scathing review of a restaurant named Alessandro's and is offered the position as head chef. Unfortunately, her colleagues weren't too fond of her, given her review and that three of them happened to be children of the previous head-chef. Monica was relentlessly picked on, until it was suggested that she hire Joey solely for the purpose of firing him, to make her colleagues realise who was the boss. The plan worked, and her relationship with them gradually improved.
It was at Ross' disastrous wedding to Emily, in the Series Four finale, that Monica and Chandler slept together. Shortly afterwards, they fell in love and tried to keep their relationship secret from their friends. Eventually, everybody found out. At the end of Series 5, Monica and Chandler decided to get married in Las Vegas, but after seeing Ross and Rachel drunkenly leave the chapel after eloping, they backed out. Instead, they moved in together at Monica's apartment and Rachel moved in with Phoebe.
In the Series Six finale, Richard came to see Monica at her restaurant and revealed that he was still in love with her and wanted to marry her. Thinking that Chandler would never want to get married, Monica considered leaving him, but instead tricked Chandler into thinking that she had left and surprised him with a proposal.
Series Seven saw Monica and Chandler organise the wedding, and they eventually got married in the series finale, even though they were almost upstaged by the news that Rachel was pregnant with Ross' baby.
At the end of Series Eight, whilst Rachel was in labour, Monica decided that she wanted to try and have a baby. But, after trying many times, Monica and Chandler had a fertility test and it was discovered that it was unlikely that the couple would ever have children naturally.
So, in Series Nine, they decided to adopt. Eventually, a girl named Erica chose them to adopt her baby.
In terms of employment, Chandler was relocated to Tulsa, Oklahoma and Monica contacted a job-hunter about restaurant openings. It was then when she was offered the job as head-chef at Javu, an upmarket restaurant in Manhattan. Monica accepted, meaning that she had to remain in New York.
It was in Series Ten that Monica and Chandler decided to leave the city and buy a house in the suburbs to raise their child, much to the dismay of their friends. Erica also gave birth to twins, even though it was originally thought that she was having only one baby.
In the last ever episode, Monica and Chandler brought home the twins, named Erica and Jack, and moved out of their apartment. They left to have one last cup of coffee with their friends.