The Reliable One

And honestly, you're the only guy I can rely on on this team.

—Burter to Jeice in Dragon Ball Abridged.

You know this character. She's the girl who is always there in the background at work, quietly doing what needs to be done without fanfare and keeps things running smoothly, sometimes just by being there rain or shine. He's the Mauve Shirt guarding The Hero's back when the Hero goes in alone to fight the Big Bad. She's the one with simple, sensible advice that her Troubled but Cute friend seldom takes. He's the Sergeant Rock that the Officer and a Gentleman depends on. And when The Captain needs data analyzed, papers filed, and problems examined, he gives it to this guy, who crunches it all and has the report ready first thing in the morning, just in time for the Captain to look at it and have a Eureka Moment.

They're The Reliable One. They're often overlooked in Ensemble Casts with Loads and Loads of Characters (particularly if it's a World of Badass with tons of flashy characters) because of his or her understated character, they nonetheless often contribute in a unique way for the good guys. Every now and then they'll tend to get A Day in the Limelight where their skills or presence come in handy even more than usual, some of their Hidden Depths get revealed, it gets shown why they frequently have such an Undying Loyalty to people around them, or where their absence makes everything around them start to fall apart.

They're often not a main character, even through they travel in the same circle as the main characters. If they are a member of the Five-Man Band, they can be any member but are most likely to either be The Lancer (being the hero's most trusted friend and go to guy) or The Chick, and is the bedrock of the team. For similar reasons in a military themed story or show they may be the Number Two. Works that go into their character frequently tend to be described as level-headed and serious if they're not an outright Only Sane Man.

If they are outside the Five-Man Band, say on the level of a Mauve Shirt, they may run the risk of becoming The Generic Guy or a Satellite Character when not given enough Character Development. In a team environment where everyone has their own specialization, (or Crippling Overspecialization, as the case may be) the Jack of All Stats may become this by default and simply being able to better adapt to circumstances.

Frequently used as a Foil to free spirits like the Cloudcuckoolander, the Manic Pixie Dream Girl, the Loveable Rogue and the Blithe Spirit, particularly if those free spirits are a major character or have a major influence on the main character. Of course, sometimes Opposites Attract and this character may find themselves in a closer than expected relationship with one of those types. The Mook Lieutenant is often a minor case of this for the bad guys.

This trope also tends to be part of a package deal for any good Cloudcuckoolander's Minder, The Confidant, The Dutiful Son, Only Sane Employee, Hypercompetent Sidekick, or Beleaguered Assistant. If they're very reliable but in a low ranking position, you often have an overlap with the Almighty Janitor.

Examples of The Reliable One include:

Anime and Manga

Jet Black in Cowboy Bebop.
Mogi from Death Note: the quiet Gentle Giant who stays with the Japanese task force through thick and thin, doing whatever is needed in an efficient manner, even when it means playing WAY against type and acting like a hyperactive kissass manager to spoiled, self-centered Elegant Gothic Lolita Misa Amane. Along the way he draws praise from figures as different as Director Yagami, L and Near for his dedication, skill in compiling reports, and refusal to accidentally give away information with an ill-timed word.
Joe Kido from Digimon Adventure.
Riza Hawkeye from Fullmetal Alchemist is often shown this way in regards to Roy Mustang.
- Perhaps also Fu for Ling.
Sayaka Yumi from Mazinger Z was The Lancer and Love Interest of Kouji Kabuto. Even if she was hot-headed, she was the one all in the Photon Atomic Research Institute could depend on, mainly The Professor -his father- or Kouji when they were battling. Jun Hono from Great Mazinger and Hikaru Makiba from UFO Robo Grendizer filled similar roles (even if Hikaru was not The Lancer).

Comics

In The Boys, the titular group is a CIA organization that keeps tabs on and confronts the often wild and sociopathic "Superheroes" of that world. It seems that Mother's Milk is often Overshadowed By the Crazy Awesome that is the damn weird Frenchman, the Ax Crazy Female, the Magnificent Bastard Butcher, or even The Woobie newcomer Hughie, but Butcher frequently says that he can't run the outfit without MM and for good reason. Hughie is the Naive Newcomer to a hopeless degree, Butcher is often blinded by his hatreds and need for revenge, and both The Frenchman and The Female are far too crazy or off in their own world for Intelligence or detective work, so when it comes to taking care of the day to day functions, MM has to do it or it won't get done and the group will collapse or become ineffective.
Snake Eyes in G.I. Joe. If he is needed somewhere or a task needs to be completed, he will be there and get it done without doubt.
Practically all of Scott's friends need to have some degree of this in Scott Pilgrim because Scott himself is notoriously flighty and unreliable, but the best example is probably Kim Pine, his long time best friend, high school ex, and arguably his biggest help in getting his head screwed on straight.

Film

Arthur from Inception. He's not the Master of Disguise, or The Charmer, or The Profiler, or The Smart Guy with the new sedative that makes the impossible possible, or the architect who creates dream worlds from nothing. He's criticized in one breath for "Having no imagination", while in the next he's praised for being the best at what he does: being The Comically Serious guy who always watches your back and makes sure you come out of things okay.
Fatal Instinct. Ned Ravine's secretary Laura Lincolnberry is a perfect example of this trope. She takes care of minor matters for him, helps him pick out his outfits and keeps her head on straight.
In the Iron Man films, Pepper Potts is this for Tony.

Literature

General Carvaille is this to Robert Epine, mixed with Poisonous Friend, in Reflections of Eterna, to the point where Robert muses in the later books whether Carvaille is the real regent instead of him.
Sam from The Lord of the Rings.
Shurf Lonli-Lokli from Labyrinths of Echo. He is also The Stoic.
Horton, as said by his line from Horton Hatches The Egg;

"I meant what I said, and I said what I meant / An elephant's faithful one hundred percent!"

Jeeves. There's a reason he has a whole trope about reliability named after him.

Live Action TV

Lennier in Babylon 5. As Delenn's aide it's his job, but he goes far above and beyond the call of duty in too many ways to count.
On Buffy Buffy calls Willow "Old Reliable," leading to a Chain of Corrections.

Willow: (sarcastically) Old Reliable, yeah, that's really a sexy nickname.
Xander: She means you're like a geyser of fun that goes off at regular intervals.
Willow: That's Old Faithful.
Xander: Isn't that the dog that the guy had to shoot...
Willow: That's Old Yeller.
Buffy: Xander, I beg you not to help me.

- Later on in the episode Willow refers to herself as "reliable dog geyser person."
Garcia the tech geek in Criminal Minds.
Bobby in Supernatural.
Teresa Lisbon (The Mentalist) always has Patrick Jane's back: Jane calls her his 'moderating influence'. Grace Van Pelt from the same series often goes unnoticed and unappreciated for the long hours she spends hunting down files and information for the other characters.
Ianto resents being this in the first series of Torchwood.
From Scrubs: Carla, the steadfast, cool-headed, keen-eyed, ever-competent, always-on-top-of-things head nurse, is the most reliable employee at Sacred Heart. Even outside of her job, she's usually the person everyone runs to with their problems.

Theatre

Benvolio from Romeo and Juliet is frequently portrayed as this.
The Admirable Crichton.

Video Games

A rare case of such character in a main role: Eddie Riggs from Brutal Legend, who is a roadie who helps his friends make history but willingly sticks to the backstage throughout the game.
There's also the APC unit in Advance Wars. It can't actually attack, but it can do a lot of useful things, like carry your infantry, act as bait (particularly in the first game) and resupply other units.
Link acts as the reliable one in several The Legend of Zelda games, but especially Skyward Sword, where his reliability allows him to help the people of the sky and retrieve Gratitude Crystals from them in order to help the demon Batreaux become human.

Web Comics

Durkon from Order of the Stick. Frequently cited by the fandom as the least interesting or developed character, (due in part to Our Dwarves Are All the Same) he's nevertheless a major part of the glue that holds the Order together, and is usually the one that its members come to for advice, help in trials of leadership, conscience, or against self-doubt. And every time he's not with the Order and they face a major obstacle, the Order starts to fall apart.
- Redcloak is this for the bad guys. He even lampshades it in one strip.

Redcloak: It's all fun and games for them, but I'm the one who has to make the magical, lightning powered trains run on time.

Web Original

Burter and Jeice seem to consider each other to be this in Dragon Ball Abridged. Leaving aside their boss, the other members of their team are a psychopathic Blood Knight who always thinks that he's in a wrestling match, and the Butt Monkey whose ability to stop time is the only useful thing about him. So when Burter needs someone by his side in a fight, or Jeice needs a favor, they call each other.
The Head Scholar of The Questport Chronicles is ostensibly just a librarian, but the Lord of the Supreme Council delegates a large amount of responsibility to her.

Western Animation

Applejack in My Little Pony: Friendship Is Magic, who is the most cool-headed, hardworking, reliable pony in the show, and actually has her own episode where it's showcased how she does too much for other ponies without thinking of herself.
In Hey Arnold!, the titular character.
In Teenage Mutant Ninja Turtles 2003, Donatello gets less attention than Mikey, Raph, and Leo. But meanwhile, he's been finding old junk and appliances and fixing them up so his family can live comfortably underground. Without him, they might not even have lights or hot water, let alone television. When their second lair is destroyed, he takes it the hardest because of all the work he put in.