So... is Alien 9 finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: February 25, 2002
Manga finished: December 19, 2002
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
Alien 9 tells the story of Yuri Otani, a reluctant middle schooler who's suddenly forced to join the "Alien Party," a group dedicated to capturing aliens that invade her school.
Yuri, who is naturally afraid of aliens, is equipped with a strange, symbiotic being called a "Borg" to help protect her.
Despite her initial fear and hesitation, Yuri finds support in her Alien Party teammates, Kumi and Kasumi, as they face the bizarre and potentially dangerous task of dealing with these extraterrestrial encounters.
The anime follows Yuri and her friends as they navigate the challenges of their duty, using their wits and unconventional tools to defend their school from alien threats, questioning whether they are truly ready for the responsibility thrust upon them.