WARNING: The following contains spoilers for Season 2, Episode 9 of Dr. Stone, "To Destroy and to Save," now streaming on Crunchyroll.
Tsukasa is by far the strongest man in the new Stone Age. Despite a strong will and stoic posture, he is not exempt from feeling emotional pain like anyone else. One of the first experiences he shared with Senku was a story about his little sister who fell ill. In the present, this heart-wrenching tale leads to a truce and perhaps salvation for Tsukasa.
In Season 1 Episode 2, Senku revived Tsukasa, the world’s strongest primate high schooler. He provided protection for Senku and Taiju and was also an excellent food gatherer. In his debut episode, Tsukasa decapitated a petrified human and made his intentions clear: to purify humanity. Before murdering the man, he briefly tells Senku about a boy who had a younger sister. The sister, a beloved Little Mermaid fanatic, was undergoing surgery. The young boy would gather seashells on the beach in an effort to make a necklace for his mermaid-loving sister, but it was a private beach. The owner beat the kid for stealing shells, and the young boy never achieved his dream.
That young boy was Tsukasa, and the resolution to the Stone Wars reveals additional details that give a deeper significance to his ambitions. In Season 2 Episode 9, Senku and the Ishigami Village must protect the Miracle Cave from an all-out assault from Hyoga and Tsukasa. Senku, Chrome, and Gen are inside the cave attempting to make a weapon of science that will once again turn the tides of the war. Meanwhile, Kohaku, Taiju, and others have their hands full with Tsukasa and Hyoga. Their aim is to stall their adversaries long enough for Senku to craft something spectacular.
Initially, Senku is stuck. Luckily, Chrome comes to his rescue with sulfuric acid found in the wreckage. Fusing it with nitric acid, Senku makes nitroglycerin, more commonly known as dynamite. A paper airplane dipped in nitroglycerin flies into a tree and explodes on impact. Tsukasa knows fighting is futile, and Senku doesn’t want to harm innocents. The two negotiate a truce and in return, Senku will try to heal Tsukasa’s sister.
In an emotional montage from the past, Tsukasa’s sister Mirai is shown in a hospital bed. She’s clinically brain dead and on life support. After Tsukasa is beaten by the beach owner, the young boy tries his hand at mixed martial arts fighting. The desperate attempt to allocate money for his sister’s hospital bills yields fame as Tsukasa grows older. He vows that he will never give up on her, but of course, the petrification happened.
Tsukasa hasn’t told anyone about his sister other than Senku. Mirai only becomes the ultimate negotiation piece because Senku infers there’s something more going on. He finds it a little weird that Tsukasa, who benefitted from fame and fortune, also wants to destroy the foundation that gave him such a fate. His background with Mirai puts Tsukasa’s motivations into perspective. In the new Stone Age, Tsukasa decides who lives or dies presumably from their virtue and value. His aim is to create a purified world where the people who used money to decide whether his sister lived or died do not exist.
Tsukasa took on the burden of deciding who gets revived as a sort of sacrifice. The world could not sustain everyone in the new Stone Age, so someone had to be a gatekeeper. His criterion is shaped by his relationship with his sister and the hardship he faced for her sake.
The miracle liquid brought Senku back to life after Tsukasa killed him in Season 1. Senku offers to use the same liquid to restore Mirai’s health and de-petrify her. Tsukasa could finally reunite with his sister and achieve his dream. In exchange, Tsukasa must honor the truce. Senku vows to keep his word, but will Tsukasa keep his after Mirai is revived?
