Akio is first seen in the Yamashiro's house where Maseo introduced him to Oliver.
While Oliver Queen was staying with his family, he asked Akio if he could use his computer. After Oliver promised to give him 5 bits of candy from his parent's unreachable stash, he gave him the laptop to use.
He later talked with Oliver when he tried to remember an event. Akio played a game of memory with him and told him he was fine.
When he was at home with his mother the Chinese Triad assassin Chien Na Wei tracked down Maseo to his house. She kicked down the door and confronted them. Tatsu told Akio to run, however Chien blocked the exit and threatened them with one of the swords of Tatsu. Tatsu threw a chair and grabbed one of the swords as well. Akio hid in a closet as he watched Tatsu fight against Chien in a sword fight. When Maseo came home he told them the white haired woman had taken Tatsu.
Akio was eventually reunited with his mother after Maseo and Oliver rescued her. After Oliver and Maseo also retrieved the Omega from Chien Na Wei, the Yamashiros planned to return to Japan. Akio and his parents were more than willing to allow Oliver to live with them until he returned to his family in Starling City. However, the four were ambushed right before leaving. Upon Maseo's orders, Oliver escaped with Akio back into Hong Kong.
The two wandered around Hong Kong for a while, with Akio nearly getting lost when he saw a man that resembled his father. They eventually decided to go to the gardens where Maseo had told Akio to meet should they ever get separated. After being chased by armed men Oliver and Akio took shelter with Mei. Akio's parents later rescued the three from an assault on Mei's home.
During the out break of the Alpha and Omega virus, Akio was one of the victims affected as he had blood coming from his mouth. Akio died in his mother's arms while Maseo and Oliver were trying to find a cure. He had been vaccinated with Oliver and his parents, however it was ineffective due to his young age. Akio's body was cremated and his ashes were stored in three urns, one for Oliver, Tatsu, and Maseo.
Akio was beloved by his parents Maseo and Tatsu Yamashiro and they did everything they could to provide for and protect him. When Oliver was forcibly staying with the Yamashiros in Hong Kong, he came to love and care for Akio in an older brotherly manner. He was the primary reason Maseo and Oliver were willing to do whatever A.R.G.U.S. ordered them to protect him from Amanda Waller. Akio was their connection to humanity and moral compass.
When Akio died from the Alpha-Omega bio weapon released by General Matthew Shrieve, his death had negative impacts on the people who cared for him. Oliver, in revenge for his death, brutally tortured Shrieve in cold blood for hours claiming that "he'd failed this city". However, he realized torturing people out of revenge, would not make him a man, contributing to the loss of his humanity. As such, Oliver vowed to become more disciplined, and only killing and torturing people in the name of justice, becoming one of the reasons Oliver would become the lethal but disciplined vigilante in Starling City known as the Hood. Oliver also honors Akio by using the words "you have failed this city" to many of his targets before defeating them as the vigilante. Maseo was unable to deal with his grief, as well as guilt, because Akio was told he would be all right feeling the last thing his son knew was a lie, left Tatsu believing himself unworthy of her love, began to view his life as a prison and joined the League of Assassins. Tatsu would leave civilization for a life of seclusion, walking down the warrior's path. When Maseo cemented his loyalty to the League, she engaged her ex-husband in combat, slaying him, giving him a mercy-kill and told Maseo he'd tell Akio they both love him.
Akio had the typical personality of a child. Despite originally being seen as a burden by Oliver, he looked up to Oliver like an older brother, enjoying playing games and other brotherly activities with him. Oliver in turn became protective of Akio during their time separated from his parents and thus shared a strong bond with him, Maseo later remarking that Oliver meant a lot to Akio. He was however sometimes cheeky and scheming as he blackmailed Oliver into giving him candy to use his laptop, but was still kind hearted and innocent.
- Multilingualism: As a native Japanese, Akio spoke that language fluently, in addition to Chinese and English.