Tsui No Sora (Endsky, TnS) is a Japanese visual novel released on August 27, 1999. Yukito Minakami is a normal guy who goes to school one day to find one of his classmates has jumped off a roof and died with two other girls from different schools. Worse still, his other classmates end up starting a doomsday cult about it and predict the end of the world is happening on the 20th. Obviously, there's more to it than that, but in the interest of not giving too many spoilers, I'll stop my explanation there.
Subarashiki Hibi ~Furenzoku Sonzai~ (SubaHibi) is something of a retelling/alternative version of Tsui no Sora, released March 3rd, 2010. There's an English version by Frontwing called Wonderful Everyday ~Diskontinuierliches Dasein~, released August 30th, 2017. Um, I'm going to be honest, the Frontwing translation kind of sucks for various reasons, but it's the easiest starting point for getting into SubaHibi.
With the special tenth anniversary edition of SubaHibi, on December 25th, 2020, they released a remake of Tsui no Sora, called Tsui no Sora Remake (TnSR, TsuiRE). It has updated graphics and and a rewritten script about 1.5 times larger than the original, as well as some new chapters.
All of them are episodic games, meaning you play the game chapter by chapter as opposed to having one "common route" and branching off of there. These are also denpa games, meaning it focuses heavily on unreliable protags and the horror of knowing something is wrong but not knowing the root cause, as well as being unable to tell what's "real" and what's not, to instill a sense of unease in the player.
I love this series of VNs, but I need to warn you: if you are triggered by anything whatsoever, do not play them. This is not a joke, they have so much I need to warn for that even as a long-time fan, I can't give an accurate list of warnings for everything they contain.
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In all seriousness, this is a difficult question to answer. She's a character in all three games, with a relatively similar role in each. She's a normal girl. She is kind of weird but undeniably human. She is an angel. She is a devil. She is the shadow. She is the light. She is the crawling chaos, an agent of Nyarlathotep. She is God. She is trying to save everyone. She is trying to bring the apocalypse. She is trying to help the main characters. She is trying to hinder them. She is the literal manifestation of Ludwig Wittgenstein's Tractatus Logico-Philosophicus. She is the true narrator. She is simply watching. She is in a dream. She is hallucinating. She is the author. She is the world. She is nothing. She is everything. She's the End Sky. She is Ayana Otonashi.
Most importantly, however, she is my little scrungly. Also she has a much more substantial role in SubaHibi, though the remake of Tsui no Sora gave her a whole chapter expanding on her.
LOOK AT HER. NOW.