His Final Choice

After Hizumi and Ayumu's final confrontation at the Tokyo Tower it quickly become clear that Hizumi isn't healing the way he used to. At the age of sixteen Hizumi has almost reached the end of his life. As a clone, Hizumi's lifespan is shorter because unreliable methods in human cloning lead to imperfections in their genes. Hizumi who was once so full of hope for the future found that when he found himself facing death he realized he wanted to die for a reason and that he didn't want to die alone.

"At the final place there would be no one to understand me, to share my pain and happiness." - Hizumi Mizushiro, Vol. 14 Ch. 71

That is why he chose to live out his remaining days as a savior to Ayumu, who would suffer the same fate as a fellow clone if nothing was done. Hizumi's cells where already deteriorating which led him to dedicate his remaining lifespan to experimental treatments that might serve to extend Ayumu's life by the time Ayumu reached the same state of deterioration. This was important because as the first successful human clones there was no tested medicine that could fix the flaws included in their genes. Hizumi's decision meant giving up the remainder of his short life to spend it in a hospital and later devoting his body to science after his death.

Hizumi does this without question. He dies two years after the end of the series at the age of eighteen. His brain functions gave out roughly half a year before his death, causing him to fall into a coma. He finally dies when his organs can no longer be sustained.

Hizumi's sacrifice allows Ayumu at least a few more years of life and this if nothing else this sacrificed allowed him to become everything he wanted to be growing up.