The anime Bleach has a supporting cast so awesome they totally steal the show on a regular basis.
We have Mr. Last Quincy Archer Ishida Uryuu, who makes his own caped costumes.
We have…oh, the entire 11th Division, who are the most crazy-ass mo-fos who ever walked the earth - but if you want me to be specific, then Kenpachi’s little pink-haired terror of a Vice-Captain Yachiru.
And Byakuya doesn’t quite count, but he gets a shoutout because he is So Hot Right Now. 
How about Arthur?
How about Dr Watson as sidekick to Sherlock Holmes?
Or Jane to Tarzan?
If we go into the realm of Classical Literature, then of course Sancho Panza is at or near the top of sidekicks/foils.
Gabrielle.
Sidekick to Xena, and lusted after by every straight male on the planet.
Artemis Gordon.
how to do spoilers:
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OK then, regarding Nuriko (spoilers for those who haven’t seen Fushigi Yuugi:
I thought it was kind of a cop-out for him to renounce being gay, start dressing like a guy, and fall in love with Miaka late in the series. I could understand Tamahome liking her, but all the other males? 
I still thought he was cool, though, and cried when he got killed by the monster (bear?) guy whose name I forgot. I cried even more than when Chichiri died.
It’s been years since I dug up my *Fushigi Yuugi * fandubs. I’d watch them again, but right now I’ve been having a bad day and don’t feel like crying.
R2-D2.
Best. Sidekick. Ever.
Gromit!
Joe Pike, the taciturn, enigmatic partner to Elvis Cole in the series by Robert Crais. Little is known about him until the most recent novel or two, but he is one hard-ass dude; totally loyal and reliable and no-one to mess with. If he has your back, you are safe.
You’ve got it backwards. Wallace is obviously Gromit’s sidekick.
[pedant]
Artemus Gordon. Artemis is a virgin goddess, or perhaps a Fowl.
No it didn’t.
My nominee- Willow Rosenberg, who also can kick my side anytime.
Spenser’s Hawk. Definitely the guy you want watching your back when your back needs watching.
A second vote for R2-D2, here.
Chewbaka is a very, very close second.
I would normally agree with Sancho Panza. The man devoted his life to Don Quixote, after all. Still, I don’t know if I’d want him at my back if I were up against The Mob.
Hawk was the person that came to mind immediately upon reading the thread title. Not only was he the guy you’d want watching your back, but he also had a definite aura of coolness about him, at all times.
I really should go back and re-read some of those books.
I cannot believe I forgot Brock Samson from The Venture Bros. I don’t care, he wins.
Did somebody ring??
User Name horn-tooting aside, ditto Artemus Gordon. What about Illya? He always boiled my potatoes…
Hmmm…what’s the current Batgirl count as? Sidekick? Partner? Free agent? Victim of a fad for Silver Age nostalgia that’ll be as embarrassing as bellbottoms and “Disco Duck” in a few years?
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