Spinel

Spinel is the main antagonist of the 2000 film, Steven Universe: The Movie, and the (former) main antagonist of its 2019 animated remake. She is Pink Diamond/Rose Quartz's former best friend of the Homeworld Gems, who was abandoned by Pink in order to turn Earth into a colony for Homeworld. After realizing Pink wasn't going to come back to her and after hearing about Steven Universe, she decided a plan to destroy Earth, all organic life, and everything Steven loves.

Appearence
Spinel's appearance is seemingly based off an old-fashioned cartoon. When Spinel be came evil, she has a dark pink complexion, magenta eyes, a small pointy nose, and magenta hair that is styled into a pair of pigtails that are messy. she has three black lines underneath both of her eyes, reminiscent of running mascara. Her clothing consists of a dark magenta top, pointy dark hot pink sleeves, dark magenta shorts, dark hot pink wrist-length gloves, white stockings and a pair of magenta knee-high boots that are pointy at the top. her eyelashes are hidden underneath her bangs. Overall, her appearance is similar to that of an old-fashioned cartoon.

Her gemstone is located in the center of her chest, cut in the shape of a heart. When she became evil, her heart-shaped gemstone became upside-down. Before she became evil, she had puffy hot pink sleeves,, and had pigtails that are shaped like hearts. In addition to her former appearance, she had a vibrant pink complexation. Her eyelashes are more visible. her boots were not pointy at that time (the time of before she became evil).

Personality
Originally, Spinel had a goofy, fun-loving and a rather innocent personality, being Pink Diamond's best friend to the point of being clingy. She greatly enjoyed her time with Pink Diamond and viewed her as her best and only friend. This friendship turned into practically undying loyalty, as she did not hesitate to wait 6,000 years for Pink Diamond when she was told by her it was part of a game.

However, upon seeing Steven's message, she realized that Pink had completely forgotten about her and started a new life with family and friends and left Spinel alone, not even bothering to send her back to Homeworld or formally tell her that Pink didn't want to bring Spinel with her. Upon realizing this, Spinel was heartbroken and brought to tears, utterly devastated with this information.

It was because of this event that she became filled with unyielding rage and hatred. Out of spite and anger towards Steven and the other Crystal Gems for their close relationships with Pink Diamond, she decided to get a drill with poisonous bile that would destroy all organic life on Earth. While fueled by rage, she still retained her humor and jovial nature, even while fighting the Crystal Gems. She took sadistic joy in knowing that Earth would be destroyed and stated that after knowing Steven, she wanted to kill him even more.

After having her innocence broken and her faith in Pink Diamond shattered, she became much more paranoid and on-guard, which was why upon hearing Steven saying that once Garnet's memories and his powers were restored, they could live "happily ever after", she viewed the remark as meaning she'd be excluded and left alone once again.

As she continued fighting with Steven, she became overwhelmingly enraged by Steven attempting to reason with her and convincing her she could change, believing that he didn't understand how she felt and that he couldn't change how she felt. She began acting purely on rage, pummeling Steven with endless punches and attacks; while doing this, she openly admitted that she had changed for the worse, going so far as to say before, she wasn't good enough for Pink Diamond, but now, she wasn't good enough at all.

Biography
Previously belonging to Pink Diamond, Spinel was abandoned by her for thousands of years, left waiting alone in the garden. After 6,000 years of waiting, Spinel finally had gotten news on what has become of her Diamond. She learns that Pink Diamond no longer exists and had given up her physical form to give birth to her son, Steven. After learning this and also realizing the fact that her Diamond abandoned her, she changed her appearance and swore vengeance on Steven and Pink's "new friends".

Later on that same day, Spinel arrived on Earth on a massive injector, that she then injected into the ground. As the injector began filling the ground with poison, Spinel began to fight against the Crystal Gems and poofed Garnet, Amethyst, and Pearl in the process with her Gem Rejuvenator. Furious, Steven manages to grab hold of her weapon and poofs Spinel with it as well.

After the battle, Steven brings the poofed gems to the temple where he learns, after the Crystal Gems had reformed, that they have lost their memory, but regained their memories back.

Later, Spinel is brought back to life, with the help of her imprisoned sister, Heart Gem, and attacks Steven, this, however, causes Garnet to regain her memories as well. Spinel then retreats to the top of the Injector and is soon followed by Steven, who then tries to talk to her. This further infuriates her, causing them to fight, until it becomes apparent she has lost her motivation to destroy him and regrets her actions up until that point. Steven, now fully reunited with his powers, comforts her and claims she can start fresh with new people to be her friends. The Injector then explodes due to the battle, and Steven saves Heart by entrapping her in his bubble alongside him, and thus trapping Spinel in his bubble, which falls into the beach below, as her stone emerges and revives back into Heart Gem. Steven then attempts to comfort her again, as Spinel decides she has created too much damage and must leave.

It is unknown what happened to Spinel afterwards, but is presumed that she is sent back into the Gem Homeworld and arrested for her cunning actions.

Pink Diamond (aka. Rose Quartz)
Spinel was Pink Diamond's former playmate and "best friend". Created solely to entertain Pink, Spinel considered herself to be Pink's best friend, and for a time, the two were inseparable. However, they gradually grew apart as Pink yearned for more responsibilities and more maturity. Oblivious to this, Spinel continued to attempt to entertain Pink, her attitude now seeming more immature in comparison. She also became clingy; upon Pink's allocation of Earth as her colony by the Diamonds, Spinel was intent on joining Pink on this new endeavor.

Very reluctant to allow Spinel to join her, Pink created a "new game" in order to get rid of her from her side, in which Spinel should wait for her return in the same spot, and never move. Spinel obliged and waited over 6,000 years for Pink's return. When Spinel finally realized that Pink had abandoned her after watching Steven's announcement on the Diamond Communicator, hurt and sorrow consumed Spinel, resulting in her taking on her current form and hunting down Steven to take revenge on Pink.

Pearl
The two were acquainted with one another as members of Pink Diamond's court, but presumably never interacted much. When Spinel and Pearl encountered each other for the first time in thousands of years, Pearl experienced shock over Spinel's return.

While the two never interact properly, it seems that Spinel holds a fair amount of resentment towards Pearl since Pink Diamond took Pearl with her to earth but left Spinel behind. This is evident when Spinel mentions it and is barely able to keep herself from screaming in anger.

Steven Universe
Spinel sought out revenge on Steven due to him being the descendant of Pink Diamond, as stated by his broadcast, and she was jealous of the fact that he, or "Pink", now had new friends that were not herself. Hurt by Pink's abandonment, she vowed to destroy Steven's new friends and the planet that Pink had loved so much. Steven accidentally poofs her, perceiving them as a threat once she has poofed Garnet, Amethyst, and Pearl.

Revived, Spinel attacks Steven and briefly holds Garnet hostage, before retreating to the top of the Injector.

Steven ascends the injector to talk to Spinel, and they fight until it becomes apparent she has lost her motivation to destroy him and regrets her actions up until that point. Steven, now fully reunited with his powers, comforts her and claims she can start fresh with new people to be her friends. The Injector promptly explodes due to the battle, and Steven saves Spinel by entrapping her in his bubble alongside him. Steven then attempts to comfort her again, as Spinel decides she has created too much damage and must leave.

White Diamond
Spinel remarked that the Diamonds saw her before, though it is unknown if this also includes White. The only known past instance of Spinel encountering the other Diamonds is when Pink was being told about her receiving her own colony by Blue and Yellow with White shown to be absent. It is possible that White was unaware of Spinel's abandonment in Pink's garden.

Blue Diamond
Along with Yellow Diamond, she was acquainted with Spinel thousands of years ago, though the degree to which they interacted is unknown. Over the course of several millennia, Blue seems to have forgotten of Spinel's existence, with her possibly being unaware of the fact that Pink left Spinel abandoned in her garden.

In present-day upon encountering Spinel she shows remorse after learning of her abandonment by Pink.

Yellow Diamond
Yellow and Blue Diamond were both acquainted with her thousands of years ago, though it isn't known how much or in which way they previously interacted. In present-day Yellow seemed surprised about "Pink's little playmate" still being around, and was possibly unaware that Pink left Spinel abandoned within her garden.

Trivia

 * Despite being the main antagonist, Spinel only has around 5 to 10 minutes of screen time.
 * It is unknown why Pink abandoned Spinel after instigating the rebellion. There are two possible reasons for that: Pink completely forgot about Spinel as she focused on the rebellion or she left Spinel behind as an act of mercy from the horror of the Gem War.
 * She represents Pink's worst action so far in the series.
 * Blue Diamond stated that Spinel is one of Pink Diamond's lost treasures, which means that there might be Gems out there like her.