Mirai is Tsukasa's beloved younger sister.
Due to an accident, Mirai was in a coma and Tsukasa refused to give up on her. To support her, he took to fighting in MMA matches to keep paying the medical bills that kept her on life support.
Even after thousands of years, Tsukasa still cared about his sister and was hopeful when Senku promised to depetrify her and states it will heal her condition. Tsukasa accepted the deal on the condition of a ceasefire. Tsukasa was extremely happy when she was fully restored and healthy after she woke up from the stone. Their reunion was short-lived, due to Hyoga. Tsukasa suffered a near-fatal wound to protect her. After Hyoga was defeated, Mirai decided to return her brother's loyalty to her and stay by his side after he went into stasis and plans to wait for him.
During his second revival, Mirai shed tears of joy at seeing him on his feet.
After being given the revival formula, Tsukasa started to free more people becoming the leader.
The inhabitants were modern humans, who Tsukasa found worthy of being freed. Using his charisma, he led them in surviving the Stone World. He cared for his followers, as he mourned the deaths of allies that died in the raid on Ishigami Village, who were killed by Hyoga.
They were loyal to Tsukasa on agreeing to surrender after Senku bartered with him.
Tsukasa and Senku start on good terms. Tsukasa is impressed by Senku's quick explanation while being pursued by a pride of lions. He also admires Senku's mind, claiming that it's the sharpest mind he has ever witnessed. Even when their ideals inevitably clashed and he was forced to kill him, Tsukasa states that had they met under different circumstances, Senku would've been his first friend.
In his fight with Hyoga, along with Senku he reflects on how fun it is to fight on Senku's side and smiles with glee having defeated Hyouga with his friend.
When Senku promised to depetrify him, Tsukasa agreed to let Senku put him in a temporary comatose state until he can heal his condition. After he returned from the Petrification kingdom, he petrified Tsukasa and then revived him, saving his life. Once Tsukasa was fully restored, he agrees to help in the conflict with Whyman.
He also joins the rest of the kingdom of science in adorning hand drawn war paint and reminiscent of scars as a sign of loyalty to Senku.
Prior to being petrified, they first met at an appearance on a psychological magic show. After the Petrification incident, Gen was de-petrified by Tsukasa Shishio and was shocked to see him again. Ironically, Gen is the kind of person Tsukasa originally wished to not revive, as he is a greedy man who cares only about his own desires. However, Tsukasa was desperate for a brilliant mind like Senku's to help him.
Gen sees Tsukasa as a dictator, who uses his charisma to win over others as he did so with his subjects and found him to be a worse kind of foe.
After Tsukasa's sister was revived and Tsukasa joined Senku, the pair have gotten on better terms. After his second revival, Tsukasa even stated he doesn't regret reviving Gen, something the mentalist smiled at.
Tsukasa and Hyoga appeared to have known each before the petrification, as Tsukasa searched for Hyoga to revive him. Hyoga become one of Tsukasa's commanding officers, as Tsukasa sent him to find the Ishigami Village and Senku. As noted by Gen, the pair were in agreement with each other on dealing with humanity's revival and made a deadly team with their skill.
After Tsukasa surrendered to Senku and got his sister back, Hyoga betrayed him, stating he waited for an opportune moment when Tsukasa was defenseless and left him severely wounded. Tsukasa would of died, had Senku not froze and later petrified him.
Soon Tsukasa realized Hyoga was once more needed to train the warriors in weaponry. To this end, he asked Senku to revive Hyoga, believing that he was better suited. Tsukasa appeared to still hold a grudge against him for the attempt on his life but promised to put it aside for humanity's future.
She was a fan of his fighting reputation prior to the petrification.
Minami became a loyal member of his empire, with him showing her the same kindness he showed his other followers.
It appears Minami has feelings for him, as when Tsukasa is revived she chooses to let him reunite with his sister first. She is pushed into talking to him by Ruri and nervously greets him while blushing heavily. He has yet to show any signs of reciprocating.
They meet after his second revival, Matsukaze could tell Tsukasa was strong and requested a sparring match, which the latter accepted. In the end, Matsukaze was defeated and surrendered, while left him in awe at his strength. Tsukasa acknowledges how strong Matsukaze could be in the future if he is given training in modern martial arts.
During the voyage to America, Tsukasa was sullen at hearing Matsukaze's past before an inebriated Matsukaze requested that he trained him. Although Matsukaze apologized for overstepping, Tsukasa agreed to this, leading to Hyoga, Homura, and Mozu bring revived.