Sonata Dusk is one of the three members of The Dazzlings living on Earth, and youngest sister to both Adagio Dazzle and Aria Blaze. She used to be a normal Siren, but was transformed into a Human on being sent to Earth.
Sonata Dusk is shown to be somewhat air-headed and carefree, similar to Pinkie Pie, and comes across as the most innocent of the three, albeit a bit naïve. For example, while The Dazzlings discussed their next step by Fish Stew Pizza, Sonata is more focused on getting lunch since there's a stuffed-crust special. Sonata doesn't pick up on Adagio's sarcasm when she says she "loves" the human world, and she speaks candidly with Jamie about her and her friends' true nature, nearly blowing their cover. In addition, she frequently argues with Aria Blaze over trivial matters. Despite this, she can be just as cruel and devious as her bandmates at times, although sometimes unintentionally.
Prior to the series, she and her siblings once lived out in the ocean, but due to Leviathan's takeover, The Dazzlings had to leave the ocean and travel inland to avoid Leviathan's wrath.
While inland, she and the others attempt to take over themselves, using their singing and dark magic to drive Ponies to fighting eachother and feeding off the negative energy. However, she was transported from Equus to Earth, being transformed into a Human in the process, and remaining there since.
Despite the setback, she and the Dazzlings became a popular music group on Earth. It is implied she went to Empire City as well, mistaking Jamie for one of the butlers that worked there.
She and the Dazzlings arrive into Beach City in their efforts to gain more magical energy. She is the second Dazzling to appear in the special, her playing in Funland, winning prizes. She alongside her siblings talk to Jamie, Sonata almost blowing their cover completely due to her air-headed nature.
She joins her sisters in their plan to take over Beach City, but during the big performance, Blue destroyed her amulet, rendering her powerless alongside Aria and Adagio. This forces her to flee the stage, and during the next performance, she admits to enjoying the song Jamie, Buck Dewey, Jenny Pizza, and Sour Cream were playing.
After her time in Beach City, she and her siblings travel around the country, until eventually they were found by Vidalia in Fairfax, agreeing to take them in as they didn't have any home. She agrees to the plan to treat Steven Universe with care only so he can take them back to Equestria. She was more than happy when Steven offered to give them a home all the same.