Kayin is a friend of Eiji Shinjo and is described as being a bounty hunter, going by the name of Storm. He was trained in sword fighting along with Eiji by Eiji's older brother Sho, until Sho had suddenly disappeared. Kayin had entered the first Toshinden tournament to avenge his foster father's recent death, who he learned had been killed by a participant who won the previous tournament. He eventually learned that Sho was the perpetrator and his friendship with Eiji was nearly shattered because of this.

Kayin was then forced to enter the second tournament after his adoptive daughter, Naru, was kidnapped by the Secret Society. At the end of the event, he managed to help bring down the fall of the Secret Society and rescued Naru from danger. Kayin had then discovered that he and Naru were being targeted by the Organization. He participated in the third tournament hosted by it and managed to help destroy the Organization. However, he realized that Naru would always be in danger because of him and he decided to leave her in a housing institute. He handed down his Excalibur sword to her and then disappeared. It is unknown of what happened to Kayin after the events of Toshinden 3, but it is likely that he may have died, because he appears as a ball of light to Naru from within her ending in Toshinden 4.

Interestingly enough, despite Kayin's nationality being depicted as Scottish and later English, he speaks with a clear American accent. It has long been rumored that his nationality was given as British so as to avoid an infringement lawsuit from Capcom, as Eiji and Kayin are virtually identical to Capcom's Ryu and Ken from Street Fighter, save for their weapons and nationalities.