Shoutout to /u/PratalMox for predicting Echo’s Origins back in 2018
So who is Echo? Saw in her in Reunion, is clearly affiliated with Overwatch in some way, and McCree seems to think Winston's New Overwatch needs her. Who is she, and why is she so important? I think it ties into the mysterious sixth founding member of Overwatch, Liao.
The grey-haired girl on the cover of the cancelled Overwatch Comic First Strike is (probably) Liao. Or at least was, the design for the character might have changed since then. And we know nothing about her.
However, judging by the cover, I would make one assumption about Liao: She was a Roboticist of some kind, and she built the Omnic that's next to her on the cover. I think that was her thing, she built Robots and Artificial Intelligences, and her understanding and knowledge of the Omnics was what led to her being tapped for the Strike Team.
So I think Liao made Echo, but more than that, I think Echo is Liao. Or a copy of her at least.
Because Echo is an "Echo" of someone else. Most of this stuff is conjecture with little or nothing to back it up, but I am certain of this. Echo is not the original, she is a copy of something, and I think she's a copy of Liao.
Liao herself is probably long dead at this point, but at some point she made an Artificial Intelligence that was a copy of her own mind, and maybe designed the body for it. It also explains why Echo projects a holographic human face. Because she's a copy of a human, that face probably feels a lot more natural for her than the faces of other Omnics.
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It also explains why Echo is important. If she's an Echo of Liao, she's the last remnant of one of Overwatch's original founders. The person she's a copy of has history with most of the major characters in this game.