Kikuri-Hime
Palace boss
The goddess of life in Shinto myth. She once mediated between Izanagi and Izanami during their confrontation in Yomi, the land of the dead.
Elemental affinity
Weak to:
Fire
Resists:
Bless
Official data only distinguishes Weak / Resist — it does not separately flag Null, Repel or Drain like some other Persona games.
Battle Mechanics
Each turn, Kikuri-Hime uses a random Arbiter's Power on the Phantom Thieves, forbidding certain actions. Defying it grants 1 Seal of Mortality; 2 seals KO the target. Arbitration - Survival: Healing/Defense skills or Highlights trigger a seal. Arbitration - Attack: Attack skills or Highlights trigger a seal. Arbitration - Support: Buff/Debuff skills or Highlights trigger a seal. If everyone obeys, Kikuri-Hime grants the whole party a damage-up effect.
Source: P5X game data files (monster table), cross-checked with in-game strings — not a community guess.