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Perhaps something went wrong when Felix was young. Oscar is horrified when he learns that his roommate was potty-trained at five months — and at six months, he was "helping others."

If there's even a speck of dust, Felix has an allergy attack. And to clear his sinuses, he makes a strange honking sound. "Everyone thinks I'm a hypochondriac," Felix says. "And it makes me sick."

• Related: Vote: TV's Greatest Neat Freak

Yet Felix is a successful photographer (portraits are his specialty), and he puts his obsessions to positive effect to occasionally bail his sloppy pal out of trouble. When Oscar gets audited, Felix finds Oscar's canceled alimony checks under his bed — in a box marked "Gambling Losses."

Indeed, Felix had redeeming qualities. When the show ended its five-year run, he remarried ex-wife Gloria and moved home.

4. Bert
Put Ernie in a bathtub with a rubber ducky and he's in heaven. But his "Sesame Street" roommate Bert has always been emotionally pinched. Bert's conical little felt head nearly explodes when he complains about his messy pal eating cookies in bed — and they don't even share a bed.

Crumbs scattered all over the apartment might explain why Bert's famously bushy unibrow is forever furrowed in boiling anger.

Bert's neat streak — and a certain stereotype about men with a penchant for tidiness — might explain the persistent rumor that the Muppet duo are lovers.

Speculation about Ernie and Bert's supposed love affair became so widespread that the Children's Television Workshop felt obliged to set the record straight in a 1993 statement:

"Bert and Ernie, who've been on 'Sesame Street' for 25 years, do not portray a gay couple, and there are no plans for them to do so in the future. They are puppets, not humans. Like all the Muppets created for 'Sesame Street,' they were designed to help educate preschoolers. Bert and Ernie are characters who help demonstrate to children that despite their differences, they can be good friends."

5. Monk
Monk returns to action this season still obsessed with his wife's murder — the only crime he's never been able to solve. Last season, he learned a few clues about her death, so perhaps he's closer to regaining his mental balance — and his badge.

Until then, his list of phobias, in order of severity, goes as follows: germs, needles, milk, death, snakes, mushrooms, heights, crowds and elevators. But as Sharona said last season, "We're working on milk."

Underpinning most of Monk's fears is the need for order and cleanliness. He refuses to sit when he's invited to a picnic. "I can't," he says. "Animals do things on the ground. Terrible, terrible things."

But it's the love of his late wife Trudy that gives Monk the strength to face the world.

Why go on? asks a baseball player who loses his wife in one episode. "To be the man she fell in love with," Monk says.

In the meantime, Monk muddles through, vacuuming and re-vacuuming until all the lines of his carpet are in a row, and investigating the murders nobody else can solve.

Perhaps it's good sometimes to be hyper-neurotic. As Monk often says, "It's a blessing … and a curse."

Buck Wolf is entertainment producer at ABCNEWS.com. The Wolf Files is published Tuesdays.