Characterization for the Hex if the Operator were sent back instead of the Drifter
I like to think about what 1999 could have been if the Lotus sent the Operator back to follow Entrati and not the Drifter. I know it makes the most sense from a character development standpoint to send Drifter, but the Operator is a much more experienced and capable (lorewise) warframe combatant with countless years of practice under their belt - it'd have been equally pragmatic to send the Starchild to 1999 to make sure whatever needs to get done in there happens and have the drifter get trained up and acclimated to the Origin system where there are other tenno to rely on to make up for inexperience and general rookie-ness.
So instead of the Hex getting a grown-ass person with magic void powers from the future, they'd get to have a war veteran trauma child - bonus points if that child is nonverbal or refuses to speak directly to them, and the Hex have to interact with them through the KIM.
And also, consider that most players from a gameplay standpoint habitually keep their Operators cloaked to prevent them from getting shot once they leave their 'frames to do whatever - canonically, the Operator and Drifter don't have a time limit on cloak, I don't think. The Hex are basically never even going to see the kid, and that could be more terrifying. Imagine seeing a puppet version of what you could be, dangling from the strings of some invisible, intangible thing with superpowers and magic lasers coming out of their hands.
Imagine mid mission Arthur turns to look at the weird and mysterious time-travelling entity and just sees a collapsed Baruuk on it's knees or frozen completely still in a weird position before it suddenly zooms about thirty meters away midair and starts straightening itself back out again in ways that joints shouldn't move in before slamming into the ground and rushing headlong into a crowd of Scaldra to 'build melee combo,' whatever the hell the demon child means by that.
Here's the meat of the post, though - the Hex (and newcomers like Flare and Kaya) can play the Context(TM) game like they do with Drifter through KIM and slowly piece together that this is a magical child soldier with a bodycount larger than all the people they've ever collectively laid eyes on, and then react in interesting ways that would lend interesting characterization.
Eleanor and Arthur would be horrified, probably, and Arthur might start talking down to the Operator because it's 'just some kid' who's clearly in over their head. Most of it's concern, but I think some small amount would be bruised pride - how could some kid be better at killing than a trained soldier?
Aoi would be heartbroken at the revelation that their new ally is a child soldier - how could anyone let that happen to them? How sick do you have to be to make children kill whoever the government pointed at with techno-organic corpses?
And Quincy would definitely be fucked up in the head for a few days after he figures it out - that could open up some opportunities for more characterization given his soft spot for kids. Would he try to keep up his aloof act, like he does with the Drifter, or immediately discard it to try and comfort a child that has already made peace with their ridiculous capacity for murder?
Amir might do something similar to Arthur, but more patronizing than hostile - he means well, but this is a kid. If Amir himself feels out of his depth in the current situation and he's an adult, then surely a child would feel even worse than he does about killing a person.
I think Lettie would be the most chill of the original Hex, though. She's jaded enough that she'd be concerned, but would probably decide that it's not really any of her business - especially considering just how talented the Operator is at mass homicide.
Velimir and Minerva could also be really interesting. On the one hand, they could see little bits of Rusalka in the Operator, but that could also lead to coddling or 'replacement goldfish'-like behavior not unlike how I imagine Amir's reaction would have been.
Flare would have been the first to know. One wayward comment from Lizzie calling the Operator demon-child or something of the like, and they'd have put it together pretty quickly after that.
Kaya would just be glad there's someone else who understands being underestimated by older people just because they're younger.