In the manga, it's explained that they basically were ordered by the Queen to live alone in their individual castles at the far reaches of the solar system while the kingdom was destroyed. When they abandoned their posts to try to find the Queen, they accidentally summoned Sailor Saturn and destroyed all of existence. That would pretty well explain their loner natures and pessimistic attitudes (wanting to save their home and all). This is also why they become a lot more social and friendly after that arc, when they learn that Saturn is not just a force of destruction and decide to raise her.
Seto Kaiba of Yu-Gi-Oh!, the Ineffectual Loner and staunch advocate of independence, self-reliance, and individual strength, which makes a lot of sense when the only way to save the world is by winning a one-on-one match in a child's card game, but no sense when The Power of Friendship rules.
Piccolo from Dragon Ball. He believes Goku is too soft and kind to be a true warrior. Later villains like Raditz and Vegeta try to replicate this effect, but they become less and less out-of-place as the story continues.
While the world(s) in Tsubasa -RESERVoir CHRoNiCLE- aren't exactly clouds and rainbows, Ninja Kurogane starts out as one of these with little regard for human life and something of a temper, but as he gets closer to his True Companions this gradually fades. In later chapters he's even sacrificed his arm to save Fay, indicating that he's fully defrosted.
Yue (Cardcaptor Sakura) is another CLAMP example. Though somewhat hard to pin, his stoicism, bitterness, inability to let go, and the fact that he has no qualms about beating the ever-living daylights out of a ten-year-old girl make him rather a grumpy bear.
Similarly Syaoran Li, an Arrogant Kung-Fu Guy who constantly gets his mystique destroyed by the cheery and kind spirited treatment of his team mates. Granted his moodiness becomes less visible through that ever recurring Luminescent Blush brought on by his "rival" Sakura's affectionate attitude.
Zelgadis Graywords of Slayers fits this trope fairly well. While the world he lives in is far from perfect, he is the most cynical member in the group of exuberant nut jobs he travels with. A group which he has tried to part ways with on several occasions, though invariably ends up traveling with anyway.
Knuckles from Sonic X. Justified in that he's a loner who guards the Master Emerald in Angel Island and has a bad temper which makes it impossible for him to cheer up. Even his own friendsdoesn't help any matters.
Usahara the rabbit from Damekko Doubutsu is not only aggressive. But he's the only character in the series who doesn't act very happy and would usually attack others when they insult him.
Naofumi of The Rising Of The Shield Hero is bitter after being betrayed and robbed at the start of his adventure, only smiling when he contemplates another's misfortune. It's a good sign that his mind is being warped by magic when he starts smiling and being friendly.
In Guardian Fairy Michel, Biam is much nastier and grumpier than the other fairies, which leads to his working for the bad guys.