Tennie

Tennie's background has been speculated by people so much to the point of outrageousness. Some say that her parents got lost in the Turbulent Sea, and some say that all generations of her family were involved in crime.
But there are also a couple of reliable theories that can be substantiated, such as how Tennie was raised by a researcher who specialized in Chaos Remains, and one who ate them at that! As long as you look more closely into any materials regarding Chaos Remains, you'll read between the lines of lab reports about a researcher who liked tasting them. This researcher's taste test records span across a significantly long period of time, lasting at least two decades, with a minimum of 1,000 variants reviewed. Not to mention, it was noted in the reports that her life had hung by a thread multiple times.
This naturally led people to wonder if someone this peculiar was even human, and they doubted she could a raise child with no issues. However, Tennie has always been an open book about her background. If anyone were to ask about her family, she'd naturally admit to the facts, referring to that researcher as her "aunt" and "guardian," all while grumbling a little about the said guardian.
"...My aunt? Why are you so curious? Are you keen on the Chaos Remain soups she makes?"
Looking at her sincere expression, the colorful, rabid-looking "cute" stuffed teddy bear in her arms, and the several massive railway guns nearby... is it really wise to keep this topic going?

Tennie's real background isn't as out of this world as rumored, but it is still indeed complicated and heartwrenching.
Before she was adopted, she mostly followed her big brother and sister-in-law everywhere, spending her rough childhood on trains and in off-road vehicles. As she got older, her existence started 'inconveniencing' the duo, so they changed their caregiving strategies, leaving her home alone for long periods of time instead.
In order to keep Tennie entertained, whenever the duo returned home from "work," they would bring her some gifts, like train engine components taken from who-knows-where, all sorts of wrenches, incomplete building block sets, and tattered machinery compendium...
Even when the duo were home, they'd usually be looking into vehicle modification and various more complicated components, hence having no time to accompany Tennie. Such was the excessively unrestricted and independent environment she grew up in.
The day that Tennie's last remaining kin got arrested by B.C.R.L., she spent it playing with building blocks at home, waiting for her caregivers to return with 'gifts.' Unbeknownst to her, the headline on "Railway Morning Post" was, "Pair of Married Train Robbers Taken Into Custody."
Shortly after, she was taken to an orphanage and was supposed to await adoption, but she remembers that one afternoon, a grinning young woman with a head of unforgettable red hair suddenly appeared before her, having already gone through the adoption proceedings and officially adopted Tennie.
Chen Shuxian—the intriguing Chaos Remain researcher and oddball that Anita staff gives a wide berth to—had, for some reason, exceedingly sympathized with this child. With that, Tennie began her brand new life with this nominal "aunt" of hers.

Tennie had once hosted a bunch of kids who came to tour the train weapon factory. Although she generally did her best to avoid such troublesome tasks, she still became moved and agreed when Anita's headquarters sent people over to convince her that this would help cultivate interests in different fields for the kids.
With her carefree personality, she quickly hit it off with the kids, becoming their newfound idol. One of them even ran over during break time and exclaimed with bright eyes, "You're so cool! Just like those auto thieves in old movies!"
Tennie completely understood why the kids would think this way. As a matter of fact, her distinguishing dressing style and huge modification factory did have some similarities with the lives of criminals depicted in movies. Nevertheless, committing crimes of any kind has never crossed her mind. Those complicated things didn't spark any interest in her; she only got to where she is now because of her dedication to the work she enjoyed and excelled at.
The words of admiration from the kids began bothering her a little. From time to time, she would recall that specific line and keep mulling over what exactly she wanted and needed to be in life. The weapon factory's workshop is exceptionally spacious, and it used to bring her joy to be here, but it only makes her feel even lonelier when her emotions are a mess.
Fortunately, she knows that she does have places to go to. She has made a bunch of trustworthy new friends who will patiently listen to all her rants and who don't mind her bluntness. Whenever she's feeling troubled nowadays, Tennie will temporarily leave the weapon factory to board The Eternal.
Should you ever come across heaps of random, disorganized train parts in the freight cars, worry not. That's just because Tennie has moved her workstation.