The reincarnation of the former YashaOh, reincarnated on Earth. He and Shulato met in elementary school and trained together in martials arts most of their lives. They were also each other's greatest rivals, eventually fighting each other in a major tournament.
During the championship match of that tournament, both of them were brought back to Tenkuukai by Vishnu. Gai ends up under Indrah's power, possessed by black Sohma. Although he was a kind and gentle person on Earth, on Tenkuukai, he becomes cold and sadistic, single-minded about killing Shulato.
In the second season, he races Shulato to find the Brahma Shakti. He actually finds it first, but the Shakti rejects him in favor of Shulato. At the end of the TV series, his Sohma merges with Shulato, and he (they) become the next Brahma. He is resurrected (maybe, depending on how you look at it) in the OAV.

Best Friend:

Hidaka Shulato