Yuya Sumii is the former leader of Team Gaim. He was originally meant to become Kamen Rider Gaim since he was offered the Sengoku Driver from Sid. He was killed by Kouta Kazuraba, who wasn't aware that it was Yuya after he ate a premature Lockseed and transformed into the Byakko Inves.
- Height: 230 cm.
- Weight: 157 kg.
After Yuya ate a premature lockseed, he transformed into the Byakko Inves, the first Berserker Inves to emerge from the Helheim Forest and attacks Kouta Kazuraba and Mai Takatsukasa, who were ironically were looking for Yuya in the Helheim Forest.
The Byakko Inves' upper body is covered with a tough shell, protecting it from most attacks, and it uses sharp claws to inflict deadly damage. It also possesses powerful muscles, enabling it to run fast and overpower Kouta when he finds his way into the Helheim Forest, as well as an energy attack that let it fire beams of energy that can slice through practically anything.
It is only after Kouta discovers the purpose of the Orange Lockseed and the Sengoku Driver that Yuya dropped when he ate the premature Lockseed, transforming him into Kamen Rider Gaim, that he is able to defeat the Byakko Inves, ending Yuya's plight.