Shinsen

was a monk who sealed Entei and Rengokuki in a cave years ago. When Kagura and the Infant find him they ultimately kill him but not before he splits the Infant in two with his spiritual powers.

Powers & Abilities
Shinsen was a legend to the point almost every monk and demon slayer has heard of him. Despite being human, his spiritual powers exceeded that of a normal human as he was able to fight Rengokuki for three full days and nights. Immediately following that, he fought Entei for three full days and nights to seal him as well. Having saved Japan from the terror those two demon wrought, Shinsen became regarded as a legend amongst anyone who lived a life fighting demons, including monks and demon slayers alike.

His powers enabled Hakudōshi to see the Border of the Afterlife even though its actually a demon graveyard and thus, no normal human should be able to see it.