There are currently five different individual Neisans that hold the title of bad end.
In Mentality, three appear in the bad ends.
1st: In Bad End 1, Neisan watches Aron attempt to escape. In Bad End 3, he falls through the vanishing step in the stairs of infinity and lands in a classroom, where he then gets tortured by a mysterious voice. In Bad End 4, Aron hears him through the wall.
2nd: In Bad End 2, Neisan dies with everyone else due to a comedic scene involving nonstop yodeling.
3rd: In Bad End 6, Neisan and the others fall down a trapdoor into a large room, where they argue with a mysterious voice. They are then left there.
In Emotivity - Morality, two appear in bad ends.
4th: In Bad End 1, Neisan and Burukku get assimilated into the darkness of Mogeko Castle.
5th: In Bad End 2, Neisan and Burukku open the wrong door and Mogeko Larvae swarmed Burukku. It is unclear what happened to Neisan, though it's possible that he faced the same fate as Burukku.
See: Neisan Buraitorando