Music is Steven Universe's secret weapon -- mainly because the show consistently cranks out some great, toe-tapping songs that are a highlight every time they show up. But the show's musical excellence extends beyond its ability to offer a good tune: It connects music to characters in some profound, interesting ways.
What Steven Universe does is assign each of its primary characters their own instrument in the show's soundtrack, reflective of their personality but also allowing audiences to aurally connect the backing music to different characters (or combinations of them).
Pearl's delicate refinery is well suited to a solo piano, Amethyst is hyperactive and fast-paced electric drums and Garnet's bass synth reflects her role as the crucial backbone of the Crystal Gems. Steven himself doesn't get a characteristic instrument (although his penchant for carrying a ukulele around ties him to his father musically, thanks to Greg's guitarist backstory), but instead a whole palette of chiptunes that represent his childlike energy. Here's a great little video by YouTuber Sideways that explores this all a bit further:
The concept of tying each character to their own specific instrument reflective of their personality in the soundtrack isn't new -- as the video explores, it's something animation has done for years -- but it's rare to see it executed so thoroughly and prominently in a modern cartoon series in the way Steven Universe does it. If anything, it takes it a step further when it explores the show's concept of fusion, with character instruments being paired off with each other to reflect the respective halves of a new fusion in their own hybrid theme songs (from piano and bass for the Pearl/Garnet hybrid of Sardonyx, to the bass and heavy electric drums of Sugilite's theme, for example).
The way the show ties character progression and personalities to its soundscape is part of what makes it so special -- and all those toe-tapping ditties that extra bit toe-tappier, when you realise the smart ways they blend character instruments together for their backing tracks.