Spinel

Spinel is the main antagonist of Steven Universe: The Movie.

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.

Due to her abandonment by Pink Diamond, Spinel is very mentally unhinged, being prone to psychotic laughter and outbursts of extreme anger when reminded even indirectly that Pink Diamond forgot about her. This is seen when her face contorts with fury after Steven asked Pearl who she was.

Despite her anger and psychosis, Spinel is somewhat trusting as she is willing to go along with Steven to stop her injector before she sees the Rejuvenator and believes he wanted to reset her. Fearing of being betrayed for a second time, Spinel immediately relapsed into insanity and decides that she hates Steven personally, before launching a preemptive attack on him.

Spinel is fueled by her vendetta against Pink Diamond and is willing and able to eliminate anyone who gets in her way. This is seen when she resets Pearl, Garnet, and Amethyst immediately after they just met, as they got between her and Pink, although she may have had every intention of erasing Pink's "new friends" that replaced her in her misplaced fit of rage. She displays a multitude of deeper emotions like when she shows remorse after Steven saves her and tearfully asks herself why she wants to hurt him and everyone else. Spinel is also mature enough to understand she will have some difficulty creating friendships after what Pink Diamond put her through, but is willing to start over with White, Blue, and Yellow.

Though being silly and goofy in her original form, Spinel has shown to be extremely intelligent and cunning in her altered state, originally defeating the Crystal Gems easily as well as understanding how the injector worked using it to kill Steven's organic form after using her rejuvenator to reset his gem, showing a great tactical knowledge