Aquamarine

"Are you "My Dad"?"

- Aquamarine asking if a person is "My Dad" (actually Greg Universe) "You know, now that I think about it, this Steven seems to know everyone on our list, the list did come from a Steven after all, maybe you can help us out? (Garnet: Just let the humans go!) Uh, no. Who are these dullards anyway? (Amethyst: We're the Crystal Gems, you chump!) Yeah, yeah. Rose Quartz's old lackeys. That was in the report, too. We're not here for any of you rouge Gems; We'll leave you totally alone, if you tell us where we can find "My Dad"! (Steven: I don't know what you're talking about!) Let's see, Yellow Diamond asked for a Connie, a Lars, a Sadie, a Mailman and an Onion I Think. Six human variations specified in a report by Peridot 5XG."

- Aquamarine revealing why she and Topaz captured Steven's friends. Aquamarine, known by herself as The Best, is an antagonist in Steven Universe.

Aquamarine is a Homeworld Gem and high ranking soldier who serves under Blue Diamond. She, along with her partner Topaz, serve as the main antagonists of the final episodes of the fourth Season and will return as antagonists in the fifth Season.

Past
Little is known about Aquamarine's past, however, it is known that she and all other Aquamarines are high ranking soldiers who serve under Blue Diamond. At an unknown point (likely after the Crystal Gems defeated Holly Blue Agate and broke Greg Universe out of the Human Zoo) she and a Topaz fusion were personally assigned by Blue and Yellow Diamond to find and retrieve several specific humans.

Season 4
In "Doug Out", Aquamarine and Topaz are briefly seen at the end of the episode approaching Onion.

In "The Good Lars", Aquamarine and Topaz are yet again seen at the end of the episode, this time approaching Sadie.

In "Are You My Dad", after Steven realizes that Onion, Lars, Sadie and Jamie have all gone missing, he finds Aquamarine, who asks him if he is "My Dad". Upon not realizing that Steven is not who she is looking for, Aquamarine flies off before he can get any answers from her. Steven then tells the Crystal Gems and Connie about Aquamarine and how she is supposedly looking for her Dad, confusing all of them, as Gems do not have parents. After splitting up, Steven and Connie manage to find Aquamarine in the woods, and, although Connie offers to help her find her Dad, she replies by angrily stating that she isn't looking for "Her Dad", but rather "My Dad". As soon as Connie states her name, Aquamarine happily flies around and tells Topaz that she "found a Connie", revealing that Topaz has trapped all of the missing people inside of her body. Topaz soon reveals that she is actually a Gen Fusion, unfusing and refusing herself over Connie in order to trap her, and when Steven tries to attack, Aquamarine takes out her Wand, forms a bubble around Steven and slams him into a tree, knocking him out. As Topaz and her walk off, Aquamarine casually states that their job is so easy, Topaz really could've done it all by herself.

In "I Am My Mom", Steven tells the Crystal Gems about Aquamarine and Topaz, terrifying Pearl, who realizes that they were personally sent by Blue and Yellow Diamond to get humans for the human zoo. To make matters worse, Garnet and Amethyst realize that, should Aquamarine and Topaz reach their ship, Steven's friends will be gone forever since Navy stole their ship. Upon getting a picture showing Funland from Connie, Steven and the Gems head there and confront Aquamarine and Topaz. The Gems try to get Aquamarine and Topaz to release Steven's friends, however, they refuse, revealing that Steven's friends are being captured because they were all on a list created by Peridot 5XG after Steven told her about his friends while she was still aligned with Homeworld, also unintentionally revealing that "My Dad" is actually Steven's father Greg, as he referred to him by that at the time.

The Gems then try to fight Aquamarine and Topaz, however, they find themselves unable to do so without endangering the lives of the humans, with Aquamarine using this to her advantage, stating that Blue Diamond never said to bring the Humans back alive, nearly having Topaz crush Jaime's skull. Steven then tells Aquamarine that he is "My Dad", which she initially finds suspicious, but soon shrugs off due to desiring to get off the planet as soon as possible. Aquamarine then starts to pilot the ship and prepares to fly back to the Zoo, however, Steven manages to use his Bubble Shield to blow apart Topaz and free everyone, frustrating her. Just as Alexandrite manages to catch the ship and everyone (sans Lars) tries to jump to safety, Aquamarine uses her wand to freeze everyone in place. Knowing the that he has no other option, Steven reveals himself as the Gem who started the rebellion and shattered Pink Diamond, Rose Quartz, causing Aquamarine and Topaz to abandon their original mission and capture Steven instead, flying off to the Gem Homeworld with him.

Trivia

 * Aquamarine's design is strangely similar to Pan-dora's
 * It is unknown if Aquamarine's primary Weapon, her Wand, is actually a Gem Weapon or not, as she is not seen pulling it out of her Gem, but rather her hair.
 * The way her weapon functions is almost identical to Syndrome's Gauntlets in The Incredibles.