"I promise you life," I replied; "but with the understanding that you serve me faithfully and none other, undertaking no business of your own, until mine has been carried to a successful conclusion." "That means that I shall have to serve you for life," he replied, "for the thing you have undertaken you can never accomplish; but that is better than lying here on a cold ersite slab waiting for old Ras Thavas to come along and carve out my gizzard. I am yours! Let me up, that I may feel a good pair of legs under me again."
Serpico as a part of Guts' new entourage in Berserk. He's the most cynical person in the group next to Guts himself, but he's wrapped in a sarcastic layer of goodness that prevents him from deserting the group, primarily for his half-sister Farnese's sake, but it goes without saying that he does care for his other companions.
McCoy from Star Trek is this, particularly towards Kirk. He's known to be a grouch and Deadpan Snarker, but he's loyal to Kirk, sneaking him aboard the Enterprise and arguing with Spock after the latter had marooned Kirk on an icy planet.
Sucker Punch plays with this in the character of Sweet Pea. She plays the part of Sour Supporter perfectly, even going so far as to Opt Out at one point. The trope play comes in when she rejoins the team and ends up the only survivor by way of the rest of the team's Heroic Sacrifices, which "redeems" her of her sourness.
In the film 1776 (and musical, both of which are Truth in Television to some extent), John Dickinson refuses to sign the Declaration of Independence, instead choosing to join the militia and fight for independence even though he believes the effort will fail.
In Sandy Mitchell's Ciaphas Cain novel Death Or Glory, when discussing difficulties getting through a mountain range, the guide Sandy Kolfax says it hardly matters since they will all die before they get there, and later Cain catches him drinking and he professes that it does not matter, as they will all die. But he does lead them. To the mountains, even. Where he dies.
In Graham McNeill's Warhammer 40,000Ultramarines novel Dead Sky Black Sun, when Uriel urges a group of Space Marines and two Imperial Guardsmen to help him, Vaanes, the leader of the group, was the last to agree, with the observation that he knew Uriel would be trouble. After they leave, he carefully ensures that Uriel knows they could all die. He finally decides that Uriel's plans can not work and that honour is not good enough to die for.
In The Silver Chair, Puddleglum professes, every step of the way, that they are certainly doomed to failure and death. His notion of cheering the children up is to tell them that they don't have to worry about something because they are likely to die first. Puddleglum is also said to be unusually optimistic and cheerful for a Marshwiggle - just imagine what the others would be like!
In Prince Caspian, Trumpkin argues against sending someone to look for Peter, Susan, Edmund, and Lucy, but when the decision goes against him, volunteers to go, because he has given his advice and now he must take orders.
In Order Of The Phoenix, Zacharias Smith, while a member of the DA, seems never to speak unless he is berating Harry's methods. Continues even after this supreme put-down:
"I don't think Expelliarmus is exactly going to help us against You-Know-Who" "I used it against him; it saved my life last summer."
Deathly Hallows Aberforth Dumbledore is almost the complete opposite of his brother. At one point, he basically says that Voldemort's already won and anyone who thinks otherwise is kidding themselves. It doesn't stop him from coming to the trio's aid when they need it, offering his pub as a meeting place for the Order, and courageously fighting in and surviving the Battle of Hogwarts.
Many of Rand Al Thor's supporters in the Wheel of Time behave like this. The man is the Chosen One (of several prophecies and groups) but none of his disparate groups of supporters trust the other ones, and many regard him as nothing more than a loaded cannon they need to further their own ends while regarding him as an idiot who messes everything up that they need to step carefully around. To be fair Rand is pretty insane by this point, and can act idiotically, but he is a lot more competent than they give him credit for and were it not for the prevalence of this trope he would have managed to do a lot more by now. Interestingly enough, Rand is actually Genre Savvy enough to try and play the groups against each other in order to get rid of some people that are hindering him. Its a safe bet that at least one of them will bite the dust but Namely High Lord Wieramon. It never works.
Grantaire in Les MisÚrables doesn't think much of Enjolras' revolution, but goes along with it anyway.
In C. S. Lewis's That Hideous Strength, the skeptic MacPhee is a valued member of the good guys' team, with the observation that he would be invaluable if they lost — but they don't know what he'll do if they win.
In John Barnes's One for the Morning Glory, they hear a ballad that ends with a young woman prisoner to the goblins, and since it's true, and they know it, they discuss rescuing her. Gorlias is as enthusiatic as any, and characters object: he's the oldest, he should be gravely warning them against it and talking of its dangers. Gorlias proceeds to discuss its dangers in a portentous tone, as if he had warned them off, but on the trip itself, he's perfectly cheerful.
In G. K. Chesterton's The Man Who Was Thursday, the Professor gravely explains the difference between himself and Syme: Syme thinks that what he proposes to do it possible, he knows it's impossible, and will nevertheless try to do it.
In NoobOmega Zell is technically this for the feminist cause in general. He's The One Guy at his workplace and does his part in its feminist association to cover up what he really thinks about women. When he gets online, his female guildmates get an earfull of his real thoughts.
In Those That Wake, Mike is this. He thinks the world sucks, he's worthless, and he probably can't do anything to change it, but he'll fight anyway.
Avon is a rare example of a Sour Supporter who's averted The Complainer Is Always Wrong at least once. He strongly argued against blowing up Star One, which turned out to cause pretty awful collateral damage for very little benefit to the rebellion, though admittedly the alien invasion didn't exactly help either.
Chiaki Tani, at first being a rebellious kid and joining the Shinkengers just so he can surpass Takeru. While he doesn't let that one bit die out, he develops more loyalty to him. This makes him being thought as The Lancer, though he later turns out to be The Smart Guy instead.
Sonya Shulen near the end of the first Suikoden, after a very hard boss fight against her. If you allow her to join, she repeatedly states that she does so in order to get the pleasure of seeing you die.
Also Flik. He's at first REALLY pissed that you got his lover Odessa killed. But he joins you anyway, and his mindset is like "Let's see if you're worthy of replacing Odessa." It wasn't until Gremio's tragic death that he starts letting off with you and eventually develops further until he's the series' Ensemble Dark Horse.
Archer from Fate/stay night; you get the feeling he only helps Shirou because his contract with Rin forces him to. As "Unlimited Blade Works" reveals, this is both utterly true but also an extreme simplification of Archer's true motives.
Cid in Final Fantasy VII joins the party only because you took and crashed his plane and constantly complains about the other party members. However, he does end up commanding the Highwind airship crew for the player and even is promoted to party leader at one point.
He calls the party numbskulls, but states that he likes the idea of fighting Shinra. That, and that he doesn't see a reason to hang around Rocket Town anymore, are the only reasons he joins.
Xan, the clinically depressed enchanter in Baldur's Gate. He is very vocal about how your quest is futile, but he helps you anyway, presumably because he sees everything else in the world as equally pointless.
Nick in Left 4 Dead 2 fits this trope to a T. He sees Coach as a foolishly optimistic and finds Ellis so incredibly naive and stupid that he won't care if Ellis gets left behind. He also constantly complains about everything, between mud from the swamps and flooding from a storm. Despite all this, Nick stays with the group because being alone would get him killed by the zombies.
Any companion with enough rivalry in Dragon Age II. Some will consistently begrudge the player's support of mages or templars, and Isabella won't believe she returned The Artifact she originally stole.
The second one is somewhat justified in that Clay knew that Lucy was actually a Templar agent all along.
Also, Lucy is the one essentially responsible for his shattered mind and his suicide, so really, he was far more respectful than he had any right to be. Still a sourpuss about the other stuff, though to be absolutely fair, being dead and crazy is enough to make anyone a little bit cranky.
Planescape: Torment features a Baatzu (a race of Lawful Evil demons) who was bound into a lifetime of good deeds by the angel Trias, to last until the angel's death. As such, he has to do everything he can to accommodate the Nameless One when he shows up on his doorstep, but he's more than happy to let him know that sure as hell isn't going to enjoy it.
Rocky the rogue from Our Little Adventure didn't really want to join the group, but he frequently does give them his best.
Karkat from Homestuck shows no respect for anyone, insults everyone he meets, gripes a lot and has grown grimly accustomed to disaster at every turn, but is near-religiously devoted to the good of his group and, after some early false starts, the human kids too, even if there are times he clearly wants to strangle them.
Pato, from M9 Girls! is not always fond of the other girls' decisions, from being treated with cosmic radiation to Karla's choice of costumes.
Sokka is cynical, started out thinking Aang could be a Fire Nation spy, and initially objected to going on this journey, but by the end of the series, through Character Development, he becomes a great leader and warrior. Lampshaded at one point when Sokka complains that apparently the rest of the team has decided he is the "Plan Guy" and he always has to be the one to come up with something to save the day. Toph points out that he's also "the Complaining Guy", but Sokka says he is actually OK with filling that role.
Katara also becomes this at the start of the second half of Book 3 when Zuko joins the Gaang; understandably, she's not happy since he's tried to capture and/or kill them for a while and betrayed Aang and Katara the one time she let her guard down. She makes it vocal that she isn't happy Zuko is there (even when he saves them from Azula) and it's only when Zuko aids her in finding her mother's killer that she finally trusts him.
Stork from Storm Hawks. He's very cynical, but has a calm acceptance of everything that happens.
Rattrap from Beast Wars is cynical, sarcastic, complains about every plan, responds to every negative turn of events with "We're All Gonna Die", and starts out incredibly reluctant to put his life on the line. Nevertheless, he's never seriously considered abandoning his friends, even jumping into the fray to try and rescue even Dinobot.
Specifically, in Under The Three Moons after saying he doesn't want to go to the school dance, Ilana and Octus make him go anyway. The same thing happened in the matter of whether he brought a date or not.
In A Family Crisis, after the trio get a distress call from Soloman, Lance brushes it off and asks for more cake. Ilana and Octus, however, think they should save him anyway even though they agree with Lance that Soloman's a jerk and doesn't deserve saving.
"How is it that everyone agrees with me, but we're still doing this?"
In the first few episodes of Recess, Vince was this. This faded, and was given to Spinelli on a few occasions.