He once worked with Ban and his sister, Himiko as a thief in the pre-GetBackers days. He used to be the driver of their group. When he turned 28 years old, he was alone with Ban in their house so that they might face his "Voodoo Child mirror" while Himiko was out to buy a cake. As a Voodoo Child, if one of two halves dies, both perish. Ban realizes this too late when his "Snake Bite" killed Yamato's other half. When Himiko came back with the cake, she saw Ban's hand covered with Yamato's blood.
Divine Design ArcIn Chapter 124, while Himiko was walking. She was surprised when shes saw her brother, Yamato.
Yamato died in the real world, however, he is alive in the upper reality made by the Divine Design. Himiko embraced him and thought he was killed by Ban. Yamato replies that Ban really killed him. He attacks Himiko and he revealed that he works for Lucifer now.
Later on, it was revealed that the Yamato that he encountered was fake after the real Yamato appeared and protects her sister Himiko.