Note: While I didn't care much about Sandrone when the Harbinger teaser trailer had dropped, my interest in her grew recently!
~ ✧ I headcanon her real name to be "Bridgitte".
~ ✧ Now, because Sandrone's codename is "Marionette", i.e. "The Puppet" instead of "Puppeteer", I wondered whether she was a human or not.
~ ✧ I now headcanon that Sandrone was born human centuries ago, but (for whatever reason) eventually had her consciousness placed into a puppet she created in her image.
~ ✧ She did this some time after Kunikuzushi joined the Fatui.
~ ✧ Just as Dottore used Kunikuzushi as a blueprint for his segments, Sandrone used him as a blueprint to create her puppet body.
~ ✧ In a leaked image where all the Harbingers are lined up, you can see that Sandrone has a key in her back.
~ ✧ Though I'm pretty sure Sandrone's body is a puppet, she doesn't need to have the key in her back. It's just there for ✨aesthetic✨.
~ ✧ After Fontaine was released, I started thinking that Sandrone used to be an engineer from the Fontaine Research Institute.
~ ✧ I think she would have been an incredibly haughty, but talented engineer dedicated to helping Fontaine.
~ ✧ She was the one who designed and produced the Gardemeks.
~ ✧ At the time, she considered the Gardemeks to be her finest work. But it infuriated her that they were wasted helping a nation that made a spectacle of the courtroom.
~ ✧ A nation with a "god" so useless that she was seen as a mascot by her own people.
~ ✧ Archons, her Gardemeks could do a better job than that fraud.
~ ✧ Something she believed in so strongly, she once tried to stage a revolt against the Hydro Archon.
~ ✧ Unfortunately, Neuvillette put a stop to that real quick.
~ ✧ Luckily, before she could go to trial, Pierro got to her to convince her to join the Fatui and create machines for the Tsaritsa's cause.
~ ✧ You cannot tell me that Sandrone isn't arrogant as hell.
~ ✧ She is the most prideful bitch in the Fatui, second to Scaramouche only because she didn't have plans to turn herself into a god.
~ ✧ In Childe's voiceline about her, he says that Sandrone looks at him like she wants to kill him, but he doesn't know why.
~ ✧ I think at one point, Childe once (accidentally) destroyed some of her work, but he doesn't remember and Sandrone is still incredibly pissed at him for it.
~ ✧ She's definitely one to hold grudges.
~ ✧ More often than not, holes herself up in her workshop and lives off of coffee.
~ ✧ Much like that modified Ruin Guard she has carry her around, Sandrone has a bunch of meks working both as her assistants and as proof of her progress.
~ ✧ A couple of months ago I posted this poll regarding whether or not I should have Sandrone be able to walk in my upcoming headcanons and her being able to walk won.
~ ✧ In spite of being able to walk, Sandrone has her modified Ruin Guard carry her everywhere, partly to show off her work, but also because she likes it.
~ ✧ Due to the fact that the Adventurer's Guild is headquartered in Snezhnaya and the similarity in their outfits, I think Sandrone is the one who created Katheryne and had a Katheryne placed in a guild in each nation.
~ ✧ Even though she and Arlecchino are both from Fontaine and hold issues with Focalors, the two of them were never close.
~ ✧ She does feel a sense of superiority knowing that while Arlecchino would still be vulnerable to Primordial Seawater, she would be safe due to her puppet body.