So... is No.6 finished or not?
It is a bit ambiguous in this case.
Why?
The anime finished before the manga:
Anime finished: September 16, 2011
Manga finished: November 28, 2013
MangaUpdates contains information about the anime adaptation and indicates that it is complete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
In a seemingly utopian city-state called No. 6, a young elite named Shion has his life turned upside down when he shelters a fugitive known as Nezumi.
This single act of kindness costs Shion his privileged status, exposing him to the harsh realities that lie beneath the city's perfect facade.
Years later, Shion and Nezumi are reunited, setting off on a journey to uncover the hidden truths and dark secrets of No. 6.
Their adventure promises to challenge everything they thought they knew about their world and the supposedly ideal society they live in.