Sidekick Song

"The secondary characters are singing a song
While the stars are snacking off-stage
It was their idea to bring us along
And now we’re hijacking this page
Of the script
We’re equipped
To steer the ship
‘Til this trippy skit ends

And by the end of this song, we’ll be best friends"

—Heidi and Susan sing "Secondary Characters" in Title of Show

In the same way that the Villain Song is the classic moment for the villain of a musical to have a chance to shine and expand upon their philosophy of life, the Sidekick Song is the chance for the hero's comedy sidekick to sing about what they want out of life - have a few laughs, spread the joy, that sort of thing. The song doesn't have to be positive but usually is. The Sidekick Song can sometimes be combined with the Villain Song for Anti Villains or Magnificent Bastards.

Often goes hand in hand with Disney Acid Sequence.

Examples of Sidekick Song include:

Film

"Make 'Em Laugh" from Singin in The Rain, possibly the most obvious example of this trope. "Moses Supposes His Toeses Are Roses", a duet between the hero and comedy sidekick, also counts.
"The Bare Necessities" from The Jungle Book. "I Wanna Be Like You" provides an Anti-Villain example (if King Louis can even be called a villain).
"A Friend Like Me" from Aladdin.
"Hakuna Matata" from The Lion King.
"Why Should I Worry" from Oliver and Company—the Dodger isn't comic relief in the end, but it's very much a "let me astonish you, young stranger, with tales of my carefree lifestyle" song.
"Under the Sea" from The Little Mermaid.
"A Guy Like You" from Disney's The Hunchback of Notre Dame.
"Be Our Guest" from Beauty and the Beast.
- With a bonus, "Human Again," in the Broadway show.
Every song from Princess and The Frog counts except the obligatory "I Want" Song and Villain Song.
"The Silly Song" from Snow White and The Seven Dwarfs, "Heigh-Ho" is another example.
"Give a Little Whistle" from Pinocchio.
"Make Your Life a Party" from the obscure Katy the Caterpillar.
One of the Cut Songs from Mulan was supposed to be one for Mushu.
- "A Girl Worth Fighting For" might count, since it's led by Mulan's closest comrades.
"No Girl's Toy" from Raggedy Ann and Andy A Musical Adventure - at least insofar as Andy is his sister's sidekick.

Musical

Spring Awakening: My Junk. It's a rare non-Angst song where the girls sing about their crushes, one guy sings about his crush on his piano teacher and another sings about his love of masturbation. Hilarity Ensues.
"Everything's Up To Date In Kansas City", "I Cain't Say No" and "All Er Nothing" from Oklahoma!! provide a set of songs for the comic relief couple. Meanwhile, "It's A Scandal! It's A Outrage!" provides one for the peddler Ali Hakeem.
"Master of the House" from Les Misérables, despite being the musical's main Villain Song, is also the musical's main moment of light relief.
- Also, Gavroche sums up his life philosophy in the plucky "Little People."
Similarly, in Oliver! "You've Got To Pick A Pocket Or Two" serves as an Anti Villain Song for Fagin.
- "Consider Yourself" sung by The Artful Dodger.
From Avenue Q, "If you were Gay", "The Internet is for Porn", "You can Be as Loud as the Hell You Want", "There is Life Outside your Apartment, and "The More you Ruv Someone" and all the Cut Songs all serve this Trope. Actually, More than 60% of the play IS this trope.
- It helps if you just pretend the entire cast is the main character. The real main characters aren't really that much bigger parts than the rest, anyway.
"The Creation of Man" in The Scarlet Pimpernel. And although the hero sings a lot of it, the hero is also his own comic sidekick.
Rent has "Today 4 You", an upbeat and bouncy song... about killing a dog, "Out Tonight", about Mimi enjoying what time she has left, and "Santa Fe", about Collins' dream of opening a restaurant out west.
"Bidin' My Time" from Girl Crazy, by the Gershwin Brothers that extolls the slacker's philosophy.
Hairspray has "Run and Tell That" by Seaweed and Inez, and "Big, Blonde, and Beautiful," by their mother, Motormouth Maybelle.
- Not to mention "You're Timeless To Me" for Wilber and Edna Turnblad.
Though the song itself isn't thought of as comical by the characters, "Kids" from Bye Bye Birdie gives the parents of the teenagers in the show a chance to say just what they think about the changing postwar culture.
"I Like Him" and "A Little Gossip" from Man of La Mancha neatly sum up (with a few interjections from Aldonza) Sancho Panza's personality.
"Dancing Through Life" from Wicked is a combination of this and a Villain Song... at least on the surface. This being Wicked, Hidden Depths abound.
"Mama Says" from the musical version of Footloose gives Ren's best friend Willard his chance to (hilariously) shine. Rusty gets her version in "Let's Hear It for the Boy".
In Guys and Dolls, "Sit Down, You're Rocking the Boat" is Nicely-Nicely Johnson's Sidekick Song.
- For extra funny, Ernie Sabella - the voice of Pumbaa in The Lion King - performs "Sit Down, You're Rocking the Boat" in the 1992 Broadway Revival of Guys and Dolls. And Nathan Lane (who voiced Timon) played Nathan in that production.
Mamma Mia! gives us Tanya's "Does Your Mother Know" and Rosie and Bill's "Take A Chance On Me".
In 13 Patrice has so many solos/songs we can't even count her. Archie has "Get Me What I Need", "Any Minute" Is a Brett/Kendra duet, "Opportunity" Is a bizzare mixture of this, the Bad Girl Song, and Villain Song. and "Bad Bad News" is this for all the boys, while "Brand New You" is this for the girls.
Next to Normal has "Ive Been" for The Caretaker Dan, and "Everything Else" for Deadpan Snarker Natalie.
"With A Little Bit Of Luck" in My Fair Lady.
- As well as "Get Me To The Church On Time"
"Not While I'm Around" from Sweeney Todd: The Demon Barber of Fleet Street.
Secondary Characters from Title of Show is the most self-referential example, as well as a sidekick duet. "I Am Playing Me" and "A Way Back to Then" are about Heidi's apprehension over her career while "Die, Vampire, Die!" is about Susan's quirky outlook on being an artist. "What Kind of Girl Is She?" is about a (potential) rivalry between the two.
Sancho, from "Man of La Mancha" has "I Really Like Him", and "A Little Gossip" could count as well.

Opera

Mozart seems to have been fond of these: both Leporello in Don Giovanni and Papageno in The Magic Flute get them. So do quite a few secondary characters in The Marriage of Figaro, which also puts the officially-a-sidekick Figaro in the lead.

Web Original

"Nobody Wants To Be Moist" from Commentary! The Musical - basically a Sidekick Song that was cut from Dr. Horrible's Sing-Along Blog and put into the commentary.
"Get Back Up" from Starship is this for Taz, although the trope is subverted in that she is Commander Up's former sidekick/mentee, and not Bug's.