Yuya Sumii

Yuya Sumii (角居 裕也) 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.

History
Originally the leader of Team Gaim, Yuya had trouble with his team during the Inves Games when teams targeted Team Gaim when Yuya himself wasn't present. As an Inves Game user, he had his own Lockseed to call on before giving Mai Takatsukasa her own Lockseed so that she can fend off for herself and the team during the Sengoku Era of the Inves Games. When Team Gaim takes a heavy hit and loses their stage, Yuya is confronted by Lock Dealer Sid, who offers him a prototype Sengoku Driver that will allow him to change the days of the Inves Games into something much more. It is presumed he was given the Pine Lockseed because it is the first Sengoku Driver given to the public as his predecessor, Takatora Kureshima used the Melon Lockseed. Yuya tells Kouta about the Sengoku Driver and tells him to meet him, but he is lured into a Crack and is drawn by the Helheim flora and eats one of the fruits, warping his mind and becoming the Byakko Inves while dropping the Sengoku Driver.

When Kouta finds Yuya's Sengoku Driver, it also converted the premature Lockseeds into the Orange and Ichigo Lockseeds. Kouta transforms into Gaim for the first time and through the help of the Priestess of Fate, he kills Yuya, never realizing he killed one of his close friends until much later. Yuya's death inspires Kouta to become something much more, someone to never resort to bloodshed, and as a result, Kouta puts blind trust in enemies and friends alike.

In the Soccer Cup event, Kougane masquerades as Yuya and reveals that he isn't Yuya before transforming into Armored Rider Mars and corrupting Kouta.

Yuya would appear in Kouta's vision with the roles switched up, having Kouta as an Inves and Yuya as Gaim.

Byakko Inves
After Yuya ate a premature lockseed, he transformed into the Byakko Inves (ビャッコインベス), 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.
 * Height: 230 cm.
 * Weight: 157 kg.

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.

Portrayal
Yuya Sumii is portrayed by Hiromi Sakimoto (崎本 大海).