Nanatose Miiku to Kyokou no Ou

At the end of 1999, a few years after the terrorist attacks that killed 1999 people, Hisato Konoe, who has the memories of 99 people, receives a postcard from his sister, "I have sent the second most powerful assassin in the world." In order to protect Nanatose Miiku, Hisato must summon the strongest man in the world.

(Source: Shogakukan, translated)