Cleanliness

A "neat freak", Monica is comically obsessive about the state of her apartment and loves cleaning (she described the dry cleaner as her Disneyland). This personality trait became progressively exaggerated as the series progressed. Examples of this tendency include: In the episode 'The One With the Embryos,' it was revealed that she categorises her towels into 11 sections, with examples being "Everyday use," "Fancy," "Guest," and "Fancy Guest."

She labels everything from dishes to photographs.

Chandler was also able to wake her up by announcing that he "probably won't spill coffee grounds all over the kitchen floor" when going for a cup of coffee.

When Ross tells her that one of his girlfriends has an incredibly messy apartment, Monica tries to go and clean the messy apartment because she is unable to sleep knowing about it.

In Season 1, she tried to act like she was a "kook" and didn't care about organization when she left her shoes out in the living room, but stayed up all night wondering if she should go get them.

In another episode it is learned that she has supplies for cleaning cars, when she doesn't even have a car, because there was a dirty car parked out front of the building. She went ahead and washed it, along with six others.

Also in one episode in Season 5 Rachel didn't want to go to an eye doctor appointment, and tried to stop Monica by knocking over a box of cereal making a mess. Monica at first pretends not to be bothered by it and they leave the apartment. However, Monica then re-enters seconds later, yelling "Chandler!" causing Chandler to clean it up.

She is obsessive to the point that she even cleans her cleaning supplies, using a dustbuster to remove dirt from a vacuum cleaner and wishing there were an even smaller vacuum to clean the dustbuster.

Competitiveness

Monica is also highly competitive, once allegedly throwing a plate in a fit of rage during a game of Pictionary, and once declaring that "rules help control the fun!". In another example, when Joey and Ross were once just casually tossing a ball back and forth in Joey's apartment, Monica joined in and turned it into a super-competitive challenge to see how long they could go without dropping it. Sometimes, however, she takes her competitive nature too far. One of the most dramatic examples of this was when Monica impulsively gambled her and Rachel's apartment during a trivia game created by Ross. When she and Rachel then lost the final question, they were forced to swap apartments with Chandler and Joey.

She also demonstrates athletic prowess, which, coupled with her competitive streak, makes her formidable against the guys, especially in Foosball. During the group's holiday in Barbados, Monica became obsessed with beating Phoebe's boyfriend Mike at Table Tennis, and the two became engaged in a marathon match lasting hours.

Sometimes, however, Monica becomes too rough. While attempting to give Chandler a sensuous massage, she hurts him to the point of his crying out. Also, in their youth, Monica broke Ross's nose when a game of touch football turned ugly in an attempt to win the "Geller Cup." The said cup was then thrown in the lake on holiday. What Ross didn't know until The One with the Football was that Monica dived in the lake to retrieve the cup. When it is revealed, the rest of the friends are confused to as what the fuss was about, Chandler saying, "Does anyone else see a troll doll nailed to a 2 by 4?"

Bossiness

Monica has a quick temper and is also bossy, and is often referred to as such by her friends. Her bossiness is best seen in her relationship with Chandler, where she frequently won the upper hand in any disputes. After they got together, she explained to him that "You don't get to win anymore." Another example was when she became Phoebe's wedding planner, and ordered the events to run on "military time," angrily tapping her watch and clearing her throat when Phoebe's speech ran too long, disguising it when the other friends looked at her.

"Fat Monica"

It is revealed early in the series that she had been obese as a child and teenager. Among the embarrassments young Monica had to endure included: When she was in Kindergarten they apparently they had to get a Junior High schooler to go on the see-saw with her.

When she joined the Brown Birds (a Girl Scouts-esque organization) she ate all the cookies she was supposed to sell, forcing her father to buy them.

When she was a goalie for the field hockey team, her nickname was "Big Fat Goalie."

Her oversized high school band uniform was a special project for the home ec classes.

When she wanted to guess the recipe of Phoebe's grandmother's chocolate chip cookie, she bakes over 20 batches of cookies. She then says that she had never baked so much cookies since 9th grade. When Rachel asks if it was for a bake sale, she says, "No, it was just a Friday night."

Monica kept the weight until she was 18, then lost it after she overheard Chandler call her "fat" at a past Thanksgiving. The writers of the series often used flashbacks to show the overweight Monica (with Courteney Cox wearing a fat suit). In one episode, when the group watched an old prom video and were surprised to see her weight, Joey yelled "Some girl ate Monica!" Monica tried to stick up for herself, claiming the camera adds ten pounds. Chandler was quick to retort, "So how many cameras are actually ON you?" In another episode where the gang remembers their past Thanksgivings, one reveals why Monica lost all the weight. In an alternate reality storyline during the show ("The One That Could Have Been"), Monica was still overweight, but nonetheless kept her occupation as a chef and still ended up in love with Chandler, who was a freelance writer.