"S.", or the Mystery Girl, is a character that first appeared in "Last One Out of Beach City".
S. bears a close resemblance to Rose Quartz, as pointed out by Steven. She has voluminous medium-length hair that is dyed a light shade of pink. She has defined lips with a gray ring pierced on her lower lip, and a slightly dark-peach complexion. She has a heavy build similar to a Quartz's and is taller than Pearl, roughly around Garnet's height. She has green eyes and wears dark gray eyeshadow and has thick eyelashes, and, aside from the aforementioned lower lip piercing, she has three gray piercings on each of her ears, with highest and middle piercing smaller than the lowest one.
In "Last One Out of Beach City", she wears a white (with a slight hue of pink) crop top with what looks like a simplified animal design, and with the sleeves rolled up. She also wears dark gray pants with the right leg having a rip near the knee, and black combat boots. She has a jacket tied around her waist; the jacket having various rectangular designs in varying shades of green similar to camouflage or flannel design. She wears black gloves and a black visor-less helmet with designs of a lightning bolt over a cloud on the right side and a sticker of a fire-breathing dinosaur skull and a sticker resembling a bat on the left side while on her Harley Davidson-esque bike.
Pearl, Amethyst, and Steven stop by the Big Donut to pick up some snacks. The Mystery Girl walks into the Big Donut and immediately catches Pearl's eye. Steven and Amethyst urge Pearl to speak to her. Pearl approaches her, but accidentally fumbles and knocks over a stack of cups before she could say anything and Mystery Girl leaves the Big Donut.
Eventually, she pulls up beside Pearl, Amethyst, and Steven on a motorcycle on the road. Following Amethyst's advice, Pearl gives the girl a look. She smiles back at her, causing Pearl to blush. The girl then drives through a yellow traffic light, while Pearl gets stuck at the red light. After a moment, Pearl drives through the red light after the girl. After the car chase with the Police Cruiser, the three hear music, and upon following it to its source, find themselves at the concert they had meant to go to. They spot her there as well. Pearl decides to go talk to the girl. Amethyst begins to worry about her, afraid that Pearl will be rejected. But Pearl's conversation with her goes well, despite being a bit awkward. Afterward, Pearl is confused, saying that the girl walked off after giving her a code on a piece of paper. Steven and Amethyst are quick to realize that Mystery Girl gave Pearl her phone number.
I asked her about her hair, and then she asked how I colored mine. I told her, "My appearance is just a conscious manifestation of light", and she said, "I know how that is", although, heh, I highly doubt it. Oh! And then I added, "By the way, I saved your planet and your species and you're welcome".
|—Pearl, "Last One Out of Beach City"|
Although not noticing Pearl at first at the Big Donut, Mystery Girl catches up to Pearl while driving, and flashes her a smile. At Mike Krol's concert in Ocean Town, she "checks Pearl out" before shaking her hand. The two discuss her dyed hair, and briefly, the Rebellion. Before leaving, S. gives Pearl her phone number.
|1. "Kindergarten Kid":||Absent||14. "The Zoo":||Absent|
|2. "Know Your Fusion":||Absent||15. "That Will Be All":||Absent|
|3. "Buddy's Book":||Absent||16. "The New Crystal Gems":||Absent|
|4. "Mindful Education":||Absent||17. "Storm in the Room":||Absent|
|5. "Future Boy Zoltron":||Absent||18. "Rocknaldo":||Absent|
|6. "Last One Out of Beach City":||Debut||19. "Tiger Philanthropist":||Absent|
|7. "Onion Gang":||Absent||20. "Room for Ruby":||Absent|
|8/9. "Gem Harvest":||Absent||21. "Lion 4: Alternate Ending":||Absent|
|10. "Three Gems and a Baby":||Absent||22. "Doug Out":||Absent|
|11. "Steven's Dream":||Absent||23. "The Good Lars":||Absent|
|12. "Adventures in Light Distortion":||Absent||24. "Are You My Dad?":||Absent|
|13. "Gem Heist":||Absent||25. "I Am My Mom":||Absent|
|1. "Stuck Together":||Absent||15. "Pool Hopping":||Absent|
|2. "The Trial":||Absent||16. "Letters to Lars":||Absent|
|3. "Off Colors":||Absent||17. "Can't Go Back":||Absent|
|4. "Lars' Head":||Absent||18. "A Single Pale Rose":||Absent|
|5. "Dewey Wins":||Absent||19. "Now We're Only Falling Apart":||Absent|
|6. "Gemcation":||Absent||20. "What's Your Problem?":||Absent|
|7. "Raising the Barn":||Absent||21. "The Question":||Absent|
|8. "Back to the Kindergarten":||Absent||22. "Made of Honor":||Absent|
|9. "Sadie Killer":||Absent||23/24. "Reunited":||Absent|
|10. "Kevin Party":||Absent||25. "Legs From Here to Homeworld":||Absent|
|11. "Lars of the Stars":||Absent||26. "Familiar":||Absent|
|12. "Jungle Moon":||Absent||27. "Together Alone":||Absent|
|13. "Your Mother and Mine":||Absent||28. "Escapism":||Absent|
|14. "The Big Show":||Cameo||29-32. "Change Your Mind":||Absent|
- According to Jesse Zuke, Mystery Girl's piercings were inspired by their World of Warcraft character.
- Zuke says they were "calling her 'Sheena' in [their] head" as a reference to the song "Sheena Is a Punk Rocker" by the Ramones. Her actual name is unconfirmed.
- Also according to Zuke's blog, Mystery Girl is a smoker, though this has never be confirmed onscreen since Cartoon Network does not allow smoking to be shown.
- S.'s phone number is 301-555-0189. The area code (301) corresponds to western Maryland, while numbers of the form 555-01XX are reserved for fictional use. The 89 in the last portion of the phone number corresponds to Zuke's birth year of 1989.
- Along with Onion and Rainbow Quartz, S. is one of the few characters with an important role that has not said a word.
- It was confirmed by storyboard supervisor and director Kat Morris that Mystery Girl would return to the show after her debut, which later came true in the episode "The Big Show".