No, Kagaya isn't a pillar but just the leader of the demon slayer corps and the head of the Ubayashiki family that was closely related to Muzan. It is the goal of every surviving Ubayashiki to kill Muzan Kibitsuji as they feel a responsibility for it due to being related to him. However, due to a sickness in the family, no man lives more than 30 years of age maximum. Kagaya has a feeble and weak body from his introduction in the series until his death.