"The Magnetite you bear points toward the star of destruction... At last... one has come whose karma is worthy of a duel to the death. In the human world, there is a saying: "Ignorance is bliss." To distance the people of my country from the cruel truth... That is the part I play in defending this land. But you would go beyond... the stars tell me so."
Tenkai (天海, lit. "Heavenly Sea"), also known as Nankōbō Tenkai (南光坊天海), a title meaning "monk of the Southern Light," was a Japanese Buddhist monk who lived during the Azuchi-Momoyama and Edo periods, dying in 1643. He acted as a consultant and aid to Tokugawa Ieyasu and several of his successors after swearing loyalty to him. The true identity of Tenkai is shrouded in mystery, as well as his age at his death, whose some believe to be as much as 130 years old. One famous theory says Tenkai was actually Akechi Mitsuhide, who is rumored to have faked his death and started a new life as a priest.
"A Tendai Buddhist monk of the Azuchi-Momoyama and early Edo periods. He was an advisor to Tokugawa Ieyasu, serving as a liason between the Shogunate and the Imperial Court. After Ieyasu's death, he suggested that Ieyasu's posthumous title be Gongen instead of Myojin, since Toyotomi Hideyoshi's posthumous title was Myojin and Tenkai believed it to be unlucky, as Toyotomi's rule ended soon after Hideyoshi's death. And so Ieyasu became Tosho Daigongen. Tenkai served the following two Shogun, and under the third Shogun, Iemitsu, he established the temple Kan'ei-ji. It is said that he also helped in city planning for Edo, incorporating feng shui into the plan."
—Shin Megami Tensei IV compendium
Tenkai is one of the National Defense Divinities and acts as the guardian of Midtown, preventing everyone from learning the Ashura-kai's "terrible secret" in Reverse Hills in order to let the humans remain blissfully ignorant of the cruel truth. He challenges Flynn to a duel to the death as the stars tell him of Flynn who seeks to learn the truth. During the battle itself, he asks two questions that pertain to Flynn's Alignment concerning his ideal Tokyo and what he plans to do once he learns the Ashura-kai's secret. He tells him whether his actions are in alignment and not to forget his resolve once he learns the truth. If Flynn answers in accordance to his alignment, he will receive a boost in attack power akin to Tarukaja. If Flynn answers with indecision to Tenkai's second question, Tenkai will become angered and all of his stats will rise, similar to the effect of Luster Candy.
As he continues to suffer heavy damage and get closer to defeat he reveals that he wants to be defeated and no longer bound to the duty he has been given, and is happy when he is beaten as the stars begin to move once more. Tenkai's final words are to tell Flynn that he wishes his next master will be someone like him. Tenkai turns into a huge slab after the battle.
To unlock him for fusion, Flynn must examine the stone that is his defeated remains with the key item, "Myoko's Disk," in his possession. His medium, along with the mediums for the rest of the National Defense Divinities, are obtained by speaking with the Master Samurai at The Hills building in Roppongi after the alignment lock.
Nanashi and company are sent to consult Tenkai on a tip from Lucifer delivered by Abe while trying to figure out how to deal with the Tokugawa Mandala. Holding Myoko's Disk which was received from Abe will allow the party to converse with Tenkai. He states that the Tokugawa Mandala is a seal that he himself placed on the region four hundred years earlier and that even though he released the seal, the Divine Powers have restored it for their own purposes. He tells the party that the seal draws its strength from jars which siphone Aether, or Magnetite, located in each Fudo temple. In order to break the seal, the jars will need to be closed. He also warns that the amount of Aether present will be dangerous to a human mind and that one with a virtuous heart may be able to resist long enough to seal the jars.
Tenkai also relates that since Nanashi has his summoning medium, that he may call on him when he becomes strong enough. Tenkai can be summoned through a special fusion of Makami, Momunofu, Kikuri-Hime and Yatagarasu once Nanashi reaches level 77. He can teach Nanashi the Mahamaon, Berserker God, Blood Ritual and Imposing Stance skills through his Demon Whisper. Tenkai benefits from learning Physical, Light and support skills.