Yu Yu Hakusho
The possessed humans were a result of the
Makai Flute and makai bugs. When the bugs entered a human's body, they became
possessed, following the will of Suzaku alone.
A few months
Puu is the spirit beast of Yusuke that hatched from
the egg Koenma gave Yusuke at the beginning of the series. Since Puu and Yusuke
are spiritually linked, Puu feels everything Yusuke does (And I think vise versa
as well). Though Puu seems to have no real purpose in the Dark Tournament Saga
other than helping Yusuke to find his courage and reason to fight (and sitting
on Yusuke's head), Puu can change into a huge bird like creature. After Yusuke's
true self emerges, Puu and Yusuke's relationship becomes much closer, as opposed
to Yusuke constantly being embarrassed by Puu's existence.