MAL's page on The Disappearance of Nagato Yuki-chan lists it as a spin-off for both The Melancholy of Haruhi Suzumiya and The Melancholy of Haruhi Suzumiya (2009).
How exactly does the new series relate to the others?
Where does it fall into the timeline?
Or is it completely independent aside from using the same characters - in which case, can it be watched without previously having watched any of the older series?
The show's existence is in itself a spoiler for The Disappearance of Haruhi Suzumiya, so anyone who hasn't seen the movie/read the book yet has been warned.
Yuki-chan is a spinoff anime based on a manga that began in 2009. The series is written by the original author of the Haruhi series, Nagaru Tanigawa, and takes place in the alternate universe created during the events of Disappearance, in which the members of the SOS Brigade are simply normal humans, and the SOS Brigade doesn't even exist.
The spin-off focuses more on the day-to-day interactions between the characters than the original series, and follows the titular Yuki-chan as she attempts to pursue Kyon, on whom she has a crush. As the series progresses, Yuki-chan develops a friendship with Kyon as she attempts to confess to him, which leads me to believe that the series is non-canon, as the following conversation takes place in Disappearance shortly after Kyon finds himself in the alternate universe (around 3:17 in the link):
"Nagato, tell me ... do you know ... who I am?"
"I do. [...] I know that you're a student in class 5. I see you every now and then. But I don't know any more than that."
The spin-off can't take place before the events of Disappearance because Disappearance Yuki only knows Kyon from when he helped her get a library card, and it can't take place afterward because
Nagato had actually used her powers to rewrite the world and take away all supernatural elements surrounding Haruhi, not just create an alternate universe, and she reverts the change at the end of the story, erasing Disappearance Yuki and the rest of the Disappearance SOS Brigade from existence.
The series is thus probably non-canon, and doesn't fit into any specific place in the timeline.
Its plot is basically separated from that of the original series, so it can be watched independently with almost full plot comprehension. However, enjoyment of the characters and their actions might requires familiarity with the source material because the much of the show's appeal stems from gap moe, in which a perceived difference or contradiction in a person's behavior or action (i.e. "gap") is found to be cute or attractive. Yuki-chan lets viewers see the normally stoic Nagato express her feelings as a normal, shy girl, and without this contrast the show turns into another "cute girls doing cute things" anime - not that there's anything wrong with those.
Here's my theory but it contains spoilers of "The Disappearance of Suzumiya Haruhi" so proceed with caution.
Perhaps "Disappearance of Nagato Yuki" is a parallel universe to the main series. Here's why: remember how Nagato gave Kyon a choice between the changed world and the original world? It was only because of that choice that allowed Kyon to fix everything to what it was again. However, what if that choice wasn't given to him? What if Nagato decided not to give Kyon the choice of reverting everything back to the original world? This TV series is the result.
I think that it's not canon and it's more of a what-if spin off where they go for what if the guy went for that girl instead or what if this event didn't happen. I've seen that in a lot of animes, sometimes it's in one ova episode and sometimes it's in 3 or more. I can put a few down like Rumbling Hearts and the Clannad series.
The reason I think this is cause in disappearance of Haruhi, Haruhi wasn't acting like her usually crazy self and was way more calm because of the change but in Yuki Haruhi is a little more crazy than even the original show. So I think it's more of a what-if thing. Forget the name of the category but there's a name for it. All I remember there the word red in the name. Also, Yuki herself is completely different too and we also forgot about Ryoko she was the same in the movie a crazy killer and kills Kyon (kind of). But in the disappearance of Yuki she's more like a normal girl who likes people and is Kyon's friend.
well everythin is completely different to the movie like how characters are and their friendships that wasn't in the movie kyon remembered and was terrified of Ryoko and Ryoko still acted like in the show with no empathy yet in this one she has empathy and is friends with kyon thats what gave me my theory
