Aoko Aozaki of Tsukihime fame was a complete grouch to everyone as a teenager in Mahou Tsukai No Yoru, in contrast to her adult self who was more or less whimsical.
Donkey Kong Country's Cranky Kong is probably video gamehood's very first grumpy bear, being an insufferable old geezer who, in between whacks with a cane, would go on and on about how much things were better in his day ("We only had a joystick and a single button! And weliked it!"), and how overblown the game is. In the animated series, when he goes on a vacation in one episode, DK suggests that he's having an awful time because there's (As Diddy puts it) 'no one to grouch at'.
Xan in Baldur's Gate. An oft-selected NPC in the original game, apparently equally for his skill in magic and his hilariously depressive and self-defeating lines.
In Mega Man & Bass, Proto Man, Mega Man's Aloof Big Brother, insists on trying a Heroic Sacrifice using a Superpower Meltdown to defeat the evil robot King, arguing it's the only way to win (never mind that Mega Man, Bass and he could probably bypass his shield collectively). It's partially successful, since it destroyed King's shield and allowed him to be beaten. It's a good thing he's a robot and could be repaired, since Superpower Meltdowns don't have the highest success rate.
Proto Man and Chaud in Mega Man Battle Network (mostly Chaud, as Proto doesn't talk much) is as far on the cynical side of the otherwise idealistic games as physically possible, lambasting Lan and Mega Man every chance he gets for not mercilessly killing every opponent he comes across under any circumstances even when there's a chance of surrender, yet strangely rather than ineffectual he saves the day every time he appears—he'll constantly pop in either just before a bad guy Mega Man had at his mercy was gonna try something dangerous or a bad guy with Mega Man at his mercy was about to kill him. In the sixth game, he actually expresses a willingness to kill Mega Man himself if he can't control his newest powers, but expresses it immediately after saving him from a villain's sword.
Francis of Left 4 Dead hates being listed here. His grouchiness is completely justified, as one of the few survivors trapped in the middle of an ongoing Zombie Apocalypse. Ironically, he's actually enjoying said apocalypse; partly because he treats the whole thing as one ongoing bar brawl, and partially because when the entire world seems to be intent on crushing hope and idealism the cynic is king. That being said, he cannot stand Louis's overindulgence of being positive no matter how bad things get mainly because he thinks Louis is being Willfully Blind to the harsh realities of the world they're living in now.
Louis: We're gonna find a sailboat right around the corner, you just watch!
Francis: Louis, if you don't stop being positive, I'm gonna sink the boat just to make you sad!
Louis: Thinking positive got me where I am, Francis!
Francis: About to die in a brick factory? 'cause I'm about to do that too! And I got to complain about shit the whole time!