Pain is the alias for the six bodies that Nagato controls through the
Six Paths Techinque provided by his Rinnegan. The Six Paths Technique,
which is only usable for those who wield the Rinnegan, allows its user
to freely control up to six bodies at once by transmitting his chakra
to them through black chakra receivers. There are other abilities
provided by the Six Paths Technique, and these can be split between
the six bodies to increase efficiency in battle. Since the bodiess
share their field of vision with the user, the latter's eyes are
reflected on the bodies'. Nagato's use of the Six Paths Technique made
it being more commonly known as the Six Paths of Pain.