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One of the Five Fated Princes, Ataru Kinniku was actually the long-lost brother of Suguru Kinniku and tried to help his sibling ascend to the throne of Planet Kinniku disguised as Kinnikuman Soldier, who was chosen by the God of Brutality.
Ataru was the blood-brother of Suguru, but was much older and chosen to be Mayumi's heir. Through the years of Spartan-like training the Kinniku royal family had designed to choose their kings, Ataru became fed-up with the relentless torture and ran away from home. He grew disgusted with himself and watched Suguru from the shadows in order to protect him and make sure he was becoming a great hero. He trained nonstop and when the Five Fated Princes appeared, he dispatched Kinnikuman Soldier, took the choujin's costume for his own, and allied himself with his brother with Team Soldier, made up of himself, Brocken Jr., Ashuraman, The Ninja, and Buffaloman.
They were given a buy into the second round against Team Super Phoenix only to lose The Ninja to Satan Cross's Triangle Dreamer technique. Ashuraman fought Satan Cross, but they both had a ring-out and were disqualified. The ceiling opened into a massive floating cube fashioned into rings on all sides. Brocken Jr. fought Prisman, Buffaloman fought Mammothman, and Ataru fought Super Phoenix. Brocken Jr. was the first to fall and nobly, he committed to a suicidal technique in order to keep his match with Prisman a draw. He fell off the ring and plummetted, with Prisman's head tucked in his legs, and smashed it into a stone statue on top of the castle which housed the first stages of the match. The sudden jolt would have caused the spine of a normal man to shatter, but Brocken uttered a final "thank you" to his friends as he died. He was the first of many to feel the sting of leaving his friends during this arc.
Meanwhile, Ataru accidentally left clues to indicate that he was not the real Kinniku Soldier for Kinnikuman Super Phoenix to leech from and form a plan. Phoenix stole the Book of Muscle Prophecy, which contained pages dedicated to the history of every choujin in history. He took Ataru's page from it and gave it to Mammothman to destroy. Buffaloman tustled with the brute until he gave it up after a Buffalo Bomb technique. Ataru and Phoenix were the only ones fighting at this point. Ataru fought a good fight. He even taught Suguru how to use the Muscle Spark technique. However, Phoenix was too strong. With a Muscle Revenger, Ataru was beaten. Suguru held his dying brother in his arms as Ataru's page was burnt. Ataru faded from the world and Suguru began to realize how serious the situation at hand really was.