From left, Judy Greer in "13 Going on 30," Rupert Everett in "My Best Friend's Wedding" and Laura San Giacomo in "Pretty Woman." (AF Archive/Alamy Stock Photo)

Sidekicks: Every romantic comedy needs one, but that doesn’t mean they’re all the same. The BFF characters may have similarities — namely a life just empty enough that they can drop everything at a moment’s notice to rehash their friend’s love woes — but they come in different packages. Some are hilarious, some are wise; some offer the glimpse of a life worth aspiring to, others show a path best avoided.

The buddies fall into a few different camps. Here’s a look at the types, and why rom-coms need them.

The aspirational figure

These characters give the romantic leads something to yearn for. One look at this sidekick’s happy relationship, filled with selfless acts of starry-eyed affection, proves that love really does exist. Conveniently, such acts take place right when the main character has lost all hope in finding a soul mate.

You’ve seen this character in:

“When Harry Met Sally.” Jess (Bruno Kirby) and Marie (Carrie Fisher) end up on a blind double date with Harry and Sally, but they only have eyes for each other. Sure, they might argue over a wagon-wheel coffee table, but who hasn’t? There’s never any question these two are meant to be.

“Someone Like You.” Just when Jane (Ashley Judd) needs a reminder that love isn’t dead, her sister Alice (Catherine Dent) has a miscarriage. Is it kind of a gross plot device? Yes, but it also allows Alice’s husband to be a doting exemplar, demonstrating how all partners should treat the women they adore.

“Forgetting Sarah Marshall.” Bill Hader and Liz Cackowski play the brother and sister-in-law of Jason Segel’s Peter. With their sound effects and dance moves, they’re goofy and a little embarrassing in that parental way, but they’re also two adorable peas in a pod.

Laura San Giacomo, center, and Richard Gere in 1990’s “Pretty Woman.” (AF Archive /Alamy Stock Photo)
The cautionary tale

If the aspirational figure shows us what the protagonist wants to be, the cautionary tale is the opposite — it’s the character our romantic leads might become if they don’t find love. And quick!

You’ve seen this character in:

“Pretty Woman.” Viv (Julia Roberts) is the hooker with a heart of gold, which makes Kit (Laura San Giacomo) the uncouth prostitute who dabbles in drugs and steals garnishes off the bar for a maraschino cherry dinner. Snag that knight, Viv!

“How to Lose a Guy in Ten Days.” Kathryn Hahn plays that girl. You know, the one who keeps getting dumped because she’s a clingy, engagement ring-hungry pile of longing. At least she has a hidden talent for playing a very funny fake therapist.

“Knocked Up.” Leslie Mann and Paul Rudd play the bickering sister and brother-in-law of Katherine Heigl’s Alison, who’s — oops! — pregnant with a stranger’s baby. Beware, Alison, this could be your future.

The sage

This character often, but not always, comes in the guise of a very young or particularly elderly person. The Yodas of the rom-com genre dispense necessary advice so that our romantic lead can see the happily-ever-after right in front of them.

You’ve seen the character in:

“500 Days of Summer.” Chloe Grace Moretz plays the kid sister of Joseph Gordon-Levitt’s Tom. Sample line: “Look, I know you think she was the one, but I don’t. Now, I think you’re just remembering the good stuff. Next time you look back, I really think you should look again.” That’s one smart kid.

“You’ve Got Mail.” Dave Chappelle nearly stole the show as Tom Hanks’s co-worker-confidant. Sample line: “I always take a relationship to the next level. If that works out, I take it to the next level after that, until I finally reach that level when it becomes absolutely necessary for me to leave.” So maybe he’s also part cautionary tale . . .

“About Time.” Dear old dad (Bill Nighy) gives his son Tim (Domhnall Gleeson) countless funny, profound pearls of wisdom, including this one: “Life’s a mixed bag, no matter who you are. Look at Jesus: He was the son of a God, for God’s sake, and look how that turned out.”

The Greek chorus

Sometimes it takes a village to make love happen. The best-friend-as-group has a couple of benefits. It proves how popular the main character is while also delivering a wealth of (occasionally conflicting) advice.

You’ve seen the characters in:

“There’s Something About Mary.” A powerhouse of funny ladies (including Sarah Silverman and Khandi Alexander) were perfectly content to talk about nothing but Mary’s (Cameron Diaz) love life. Oh, and sex toys.

“The 40-Year-Old Virgin.” The comedy benefited from an eclectic bunch of dope-smoking co-workers who weren’t that much more successful with the ladies, yet were still happy to coach Andy (Steve Carell) through his first sexual encounter.

“Bridget Jones’s Diary.” Bridget (Renée Zellweger) has a whole crew of besties who may not give great advice but sure know how to show a girl a good time — not to mention embarrass her when her one true love unexpectedly drops by for dinner.

The comic relief

Sometimes the lead characters simply aren’t up to the task of delivering the funny. In that case, a talented supporting player with spot-on comic timing is required by law to step in.

You’ve seen the character in:

“My Best Friend’s Wedding.” Did anything even happen in the movie other than Rupert Everett performing a transcendent rendition of “I Say A Little Prayer”?

“High Fidelity.” Without Jack Black, this ode to music would have been a touch too melancholy. Luckily, the comedian was there to make Cosby sweater jokes (back when those kinds of jokes were funny), quibble over the existence of the word “the” in an album title and flaunt the most inefficiently hilarious way to flip someone the bird.

“Notting Hill.” What would the movie be without Rhys Ifans’s T-shirt collection and buttcrack peekaboos? A lot of sad-eyed longing.

Jennifer Garner, left, and Judy Greer in 2004’s “13 Going on 30.” (AF Archive/Alamy Stock Photo)
Judy Greer

She’s played so many supporting characters that she wrote a book titled “I Don’t Know What You Know Me From: Confessions of a Co-Star” and starred in the Funny or Die short “Judy Greer Is the Best Friend.” She’s been second banana to a bunch of Jennifers, including Garner (“13 Going on 30”), Aniston (“Love Happens”) and Lopez (“The Wedding Planner”).

She was also Heigl’s pal in “27 Dresses,” a movie about a woman who’s always the bridesmaid. How do you like that for irony?

Greer doesn’t always play the same type of best friend, but she’s consistently wonderful at whatever best friend she plays. Which only leads us to wonder: Hasn’t she paid her dues? It seems well past time for Greer to get a sidekick of her own.