To add to a confusion, Noriko says that they last saw each other almost 3 years ago.
Maya did casually dropped the word "timelines" the first time Noriko appeared, so...
In a "Abyss" event, when Sensei blacked out before Yumi, an unknown entity tells him that Yumi looks scared, just like Sensei was, when he was with "the world" (Sekai).
I believe this is her.
On the side note, I've recently remembered the manga I read long time ago, even found it's name again - Classmate, Kamimura Yuuka wa Kou Itta. It's has a high school setting, everybody is a robot (or rather complex, human-like androids), including MC, except for one human girl, who they all programmed to ignore. It turned out that before they were all put in high school, they lived in different simulations, had different lives names and duties.
This game recently start to remind me of this manga. I thought it was suspect, when Maya made direct parallels between how computer works with memories and how normal brain does, and then she immediately insinuated that girls work with memories they have much like computer do. Also in the Yumi's event Abyss, a Purple Smile...thing said that it knew Yumi by different name. In that same manga I was talking above, some androids were put in war setting, which reminds me of Miku situations, her ptsd and "night-vision" references, that keeps popping out.
Would be hilarious if it turned out that Selebus used this manga has one of inspiration sources.