Ayumu is stoic, carefree and intelligent, however he is also introverted and has a lot of self-doubt. He fears that he cannot surpass his older brother, and therefore doubts himself to the extent where he believes he can do nothing but act like his brother. When Hiyono tries to cheer him up or talk to him, he usually ignores her. Ayumu confesses to Hizumi that he forgets Hiyono's name at times, but is shown later to remember it when important. He also cares for and worries about Madoka a lot. He is a lot more violent in manga as he has shown no problem using guns on others like Eyes and Kanone
Ayumu has spiky brown hair and a piercing in his right ear. He is often seen in his blue school uniform and has green eyes in the manga and brown in the anime. Ayumu looks like a younger version of Kiyotaka. He doesn't show it but Ayumu has a tattoo of a puzzle piece on his forearm. In the finals chapter, he was shown to have rash-like markings around one of his ears, most likely as a result of his body failing.
Ayumu was alienated by his parents. He was a clone created for Kiyotaka as a type of "insurance" by his parents after Kiyotaka started playing piano, so if anything bad were to happen to Kiyotaka, Ayumu would be used as a "guinea pig" and undergo treatment to find what was suitable for Kiyotaka's body. However, once Kiyotaka announced that he was quitting piano, his parents found no use for Ayumu and was often ignored. Kiyotaka also stole Ayumu's first love (Madoka).