He looks exactly like the real Neisan, but with black eyes.
In Emotivity - The Prologue, Neisan was chased by Nightmare Neisan in his dream. During the chase, the Nightmare asked Neisan cryptic, and bizzare questions. Nightmare Neisan then slashes at Neisan, ending the dream.
Nightmare Neisan is a very enigmatic, cryptic, bizzare, and twisted version of Neisan.
Nightmare Neisan's origin is unknown, but may be revealed at some point in the future.