Want to know how Steven Universe, the character, was created? Read this!
Ok, so, I understand Rose Quartz, Steven's mother, and Gregory Universe, Steven's dad, 'made' Steven, but I think I know why Rose 'disappeared' during that.
Rose said in a video that she and Greg made for Steven to watch that she was going to disappear, and that she will pretty much become him.
Rose says here "...and every moment you love being yourself, that's me loving you, and being you..."
You know Ruby and that other blue chic? They fused in an episode, idk which though, but here's a link to a YouTube video showing this shizzam.
The two created Garnet, by fusing.
Still not convinced? "Oh yea? Well, what about Pearl and Amethyst?"
Well, I honestly don't know. And I don't care, but seriously...