Have a gem:

Name: Charoite
Gem Type: ???(Unknown)
HM or CG: Homeworld
Gem Duty: On homeworld, gems like her are entertainers. They are the announcers, the ones who calm the crowds, which relates to Charoite's powers.

Powers: Charoites are born with one or both of these powers: The ability to examine the past of others, and the ability to alter the opinions of others with their voices. Ones that only have the former are smashed and used to make jewelry for the survivors. It's homeworld's way of saying "you aren't an abomination, good for you." Our Charoite has both abilities.

Additional Information: Charoites are very rare gems, and so, are shared carefully amongst the diamonds. They are also pretty physical weak, and, if in battle, rely on mental power.

My idea of how she might meet Steven is that she's thrown onto a ship by traitors to homeworld and sent off to space, landing on earth. Either way, she is not comfortable on these foreign grounds.
Eventually, through learning about Earth, and through Steven, she and Steven both discover that their views on the war are very biased in different ways. They bounce off each other, as they both have never met Rose, and have both had her built up to be this powerful person; for Steven, as a saviour who fought for the sake of others; for charoite, a demon who wants to destroy society as she knows it.

Eventually, charoite decides to join the crystal gems, but she makes it clear that, like peridot and lapis, she is not fighting to continue what Rose started. Her added line to the song describes it best

'I will fight for the sake of Homeworld, because deep down I know it's still my home!'

She says not for homeworld, but for its sake. She believes, much like steven, that any one can be a good person. But unlike him, she sees the world in shades of grey. No one is entirely good, no one is entirely bad. Not the crystal gems, not the dianonds, not homeworld, and certainly not her. And she tries to teach this to steven, the best she can.

She wants to unite homeworld as a better place, where size and hierarchy doesn't matter. She loves homeworld, she loves her home, and she fights for its sake, and for everyone in it's sake. For every gem, from a peridot to a bismuth to an some important thinker to a diamond to a court gem like her.

She now knows that homeworld will not survive without change. And that change is what she will fight for, Steven.

"I know that neither of us know your... Mom, Steven. But there's a phrase I always tell myself when I feel bad for bending people's minds:  'Someone's gotta do it. Somebody's gotta be the bad guy here.' Maybe that's what was going through your mother's head?"

"I know you don't want me to do this, my luster. But someone's gotta do it."

"My diamond, I am not trying to fight you! I respect you, and I don't want to fight you at all! You know that. But I do truly think that homeworld needs to change. Think about it, my diamond. A war was already lost, but we did not learn from it at all. If we keep going the way we are now, there will be another one. One is already coming, I believe. Think about how many times I had to use my powers in your court. We need to change. If there is another war, Homeworld might not survive. Now, I understand, that you are probably very angry with me at this point. You may not even be listening to this part of the message. Maybe you've stopped it, smashed it, had it taken away. But let me tell you one more thing.
Until everything is settled, I temporarily denounce my loyalty to you.
I'm so, so, sorry, my diamond.
Goodbye."