Throughout the game, there are several NPC Revenants that are references to Team Ninja's works:

Shibasawa Kou - The game's producer
Samurai_Gaiden - Ninja Gaiden
Shadow Warrior - The European name of Ninja Gaiden
RebootRygar - Rygar
GITAROO-MAN - Gitaroo Man
theJaquio - The main villain from Ninja Gaiden
Holy_San - Holy Vigoor Emperor from Ninja Gaiden
Genshin07 - Genshin from Ninja Gaiden II
LOA.test.subject.1 - Lords of Alchemy from Ninja Gaiden 3
IreneIsMyHero - Irene Lew from the original Ninja Gaiden Trilogy
Dr. Smith - Dr. Walter Smith from Ninja Gaiden (NES)
oinin_AYANE - Ayane from Dead or Alive
nukenin_KASUMI - Kasumi from Dead or Alive
ZACK - Zack from Dead or Alive
GARI - Dr. Gari from Tecmo Secret of the Stars
KarmicPunishment - The curse from Ninja Gaiden 3
Hajinmon4Life - Ayane's fighting style, Hajin Mon
dropped_by_izuna - The Izuna Drop move from Ninja Gaiden and Dead or Alive