Jikokuten is one of the four Devas protecting the barrier that separates the human world from the demon world. Before the events of the game, Amane Kuzuryu managed to defeat him using the power of Jezebel, weakening the barrier enough for the Demon Summoning Program to be used with ease.
On Yuzu's 8th Day, Gin uses his demon companion, Take-Mikazuchi, as a temporary replacement for Jikokuten so as to restore the barrier separating both worlds.
Devil Survivor 2
"Jikokuten, or Dhrtarastra, is one of the Four Heavenly Kings in Buddhist lore. He dwells halfway up Mt. Meru in the center of the world and protects the East under the command of Sakra. He helps maintain the security of the nation. He is commonly shown wearing armor, with a sword in hand and an angry expression on his face."
—Devil Survivor 2 Compendium
Jikokuten is first summoned when Dera Daka attempts to use Nicaea on the 6th Day. However, Dera Daka is unaccustomed to the sheer might of the demons he has summoned and is killed after failing to seal the contract. The protagonist has to help him defeat the demons, including Jikokuten, alone. Dera Daka's final wish to the protagonist is to keep the events of their meeting secret from Airi.
Finishing this battle unlocks Jikokuten for fusion.