So... is Phantom: Requiem for the Phantom finished or not?
It is a bit ambiguous in this case.
Why?
The anime finished before the manga:
Anime finished: September 25, 2009
Manga finished: January 27, 2010
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 crime-ridden America, a shadowy organization named Inferno controls the underworld using a deadly assassin known as Phantom.
An unsuspecting Japanese tourist stumbles upon one of Phantom's assassinations and finds himself caught in Inferno's web.
Brainwashed and stripped of his memories, he is given the codename Zwei and forced to become an assassin himself, struggling to survive in a world of violence and betrayal.
Zwei must navigate this dangerous new reality while grappling with his lost identity and the possibility of escaping Inferno's grasp.