So... is Ninpen Manmaru finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: March 28, 1998
Manga finished: March 12, 1999
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
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
Ninpen Manmaru is about a young ninja-in-training named Manmaru.
He attends a unique ninja school at Nenga, where the students are anthropomorphic animals like foxes and raccoons.
They're all trained in the ways of assassination by an even more unusual bear ninja master.
Manmaru's training is quickly complicated by an ongoing feud between the Nenga school and rival ninja clans composed of monkeys (Koga) and dogs (Iga).
On top of that, Manmaru faces internal struggles as he's tempted to join the Dobe, a group of disillusioned students who have abandoned the ninja path.
Essentially, Manmaru's journey is a coming-of-age story filled with rivalry, temptation, and the challenges of learning to be a ninja in a world populated by animal ninjas.