Steve the Pirhana Plant in The Movie of Bowser's Kingdom. Why After Hal insulted him one too many times, Steve told Bowser they had betrayed him, setting the events of the movie in action.
In Red vs. Blue, Sheila is pretty nice and even-tempered, especially in comparison to the rest of the cast. She's also the installed AI in a tank.
Another AI example is Delta. Mostly emotionless, he's nonetheless shown subtle and very touching dedication to his partner York, even insisting on staying beside him into death. He's later revived, and partnered with South Dakota, who proceeds to try using him as bait to let herself escape. When South is captured, it's Delta that recommends that she not be given further opportunity for betrayals.
Caboose is another example. He started out as an idiot and only gotdumber. However, he is by far the nicest member of the main cast; he cares more about making friends than fighting (unfortunately, when he does fight, generally he's a danger to his allies). However, when he worked himself up into a fury, aside from launching into a series of nonsensical one-liners ("Your toast has been burnt and no amount of scraping will remove the black parts!"), he defeated two armies of flag-obsessed zealots. And later quite a number of Texbots all on his own.
Add North Dakota to the list as well. He's easily the nicest and friendliest of the Freelancers, to the point where he got the AI he did specifically because he has a nurturing personality and is the Team Mom. But he's still one of the top Freelancers, capable of mowing down an army of Mooks and expertly dual-wielding sniper rifles.
Catbug is adorable, innocent and loves to play with anyone and everyone. He also shaved Danny's eyebrows (and possibly Chris' hair, too) upon acquiring an electric razor in "Browser Fail", and was revealed to have killed Jelly Kid upon their first meeting in "Jelly Kid Forever". (To be fair, he *is* part-cat, and happily justified his actions as being a present for Danny...like any cat would.)
RWBY takes place at a military school in a World of Badass, so any 'nice' character probably fits this trope. Special mention goes to Pyrrha, who introduces herself by saving a total stranger from plummeting to his death with her excellent spear-throwing skills and power over magnetism.