Yamato Takeru, the Brave of Yamato, is the title given to a legendary prince named Ōsu, who defeated his enemies valiantly in his father's name. When he died, his spirit turned into a great white bird.
"Yamato-Takeru no Mikoto was one of the greatest warriors of Japanese lore. He was said to be half human, half god. Around six feet tall with a stern face, he prided himself on his strength. He was ferocious and killed his older brother in a trivial argument. His father, Emperor Keiko feared his son and sent him to war at the western front, and then on an expedition to the east after his successful return. On his way east, he met his aunt, who gave him the divine sword Ame no Murakumo no Tsurugi. It is said that when he was attacked by fire at Sagami no Kuni, he cut the grass and was able to escape alive. From this, the Ame no Murakumo also became known as the Kusanagi no Tsurugi (grass-cutter sword). He eventually died of a curse from a god of Mt. Ibuki, but he turned into a great white bird and flew away."
—Shin Megami Tensei IV Profile
Yamato Takeru is one of the National Defense Divinities enslaved by Tayama. He is summoned by his master at Camp Ichigaya to defend the Yamato Perpetual Reactor from intruders. On Walter's route he is faced as a boss. Introducing himself, he ask them not to think ill of him since he attacks on the orders of his master. He sees their great skill during the battle, but states they carry different burdens, comparing them to a wild beasts that carries no weight upon them, while he holds a nation on his shoulders and protects its people. He will be angered if called Tayama's slave and reveals that he views Tayama as scum, but since he can't disobey him he will serve him to the best of his ability.
As he takes more damage he will ask another question, asking if they truly considered the consequences of their actions as they took this course. Later in the battle he will be shocked at their power and will shudder at the thought of chaos thriving in the nation where harmony was valued, and will retort to Walter's statement that a nation is one where the people are united. As he is defeated, Yamato Takeru will proclaim that history has sided with them and that this is the fate of the nation, crumbling afterwards.
If the protagonist sides with Jonathan however, Yamato Takeru will have been defeated by Walter beforehand and will never be encountered. He is seen dying next to Tayama's corpse in the room with the Yamato Perpertual Reactor, giving a pained laugh at the fool who believed he was the King of Tokyo and met a suitable end.
To unlock him for fusion, Flynn must examine the stone he drops with the key item, "Yaizu's Flint," 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.