Galaxite

She is a horrible gem, really. Just the worst. She is a General in the Homeworld military and is quite the little zealot. She conquers galaxies and destroys her opposition with an iron fist. She is also a severe classist and finds gems like our heroes disgusting because they intermingle with one another-

_{Rebecca Sugar: "There is a lot to the world of the show that is still hidden! The show is all from Steven’s point of view, and there’s a lot going on on the Gem Homeworld that he doesn’t know about at all, and a lot of rules that exist there that the Gems we know don’t follow! Amethyst doesn’t know a lot of this either, since she was created on Earth. The Guide Book contains everything that Steven knows at the moment, plus a few logistics about his birth that might be a bit outside of what he’s been told. Steven’s going to learn a lot more soon on the show, about what each of the Crystal Gems are supposed to be… it’s very different than the way they’ve been living on Earth. It is very unusual for Gems like Garnet, Amethyst, and Pearl to be living and working together the way they do, almost as unusual as it is for Steven to exist!"}

after reading this quote from her I got a picture of Homeworld as a place where there are multiple classes and some are more elite than others. And they definitely don't mix. Galaxite wholeheartedly agrees. 
Overall she is just downright cruel. She enjoys inflicting pain and relishes in her job. Her other quirk is that she is "manic-depressive". One minute she is on top of the world and the next she is depressed and melancholy. Unstable is a good descriptor for Galaxite. 
Her weapon is a battle axe (get it... gal-AX-ite.... ha) but she's also got a power crystal that was given to her by her superiors. She wears it as an insignia. It produces quite a bit of radiation and when she charges it up it can be quite dangerous to organic life. It boosts her already impressive strength and powers. 
Please do not use or repost without my permission
Also- fun fact, she's only slightly taller than Amethyst (she is a tiny tiny ball of rage)
Art (c) withery.deviantart.com
Steven Universe (c) Rebecca Sugar