Spinel

Spinel is the main antagonist of Steven Universe: The Movie. Vengeful on her former friend Pink Diamond and Steven Universe, she brings a giant doomsday weapon with the intent to destroy Earth in 48 hours until being defeated by Steven.

Biography
TBA (No spoilers!)

Personality
Originally, Spinel had a goofy, fun-loving personality, being Pink Diamond's best friend to the point of being clingy.

In stark contrast to her original personality, upon being abandoned — and learning that Pink is gone (and that Steven has taken her place) and of Pink's other friends — Spinel is bitter, full of resentment towards Pink Diamond, Steven Universe, and all of their affiliates.

Role in the series
In Gems vs. Elements, it's revealed that Spinel was an unknowing pawn for Grand Advisor Jerktonium to try and eliminate a threat to his plans of exterminating all Gems on Homeworld, seeing them as a defective race created against the will of the Matter Councilors.