Keigo Masaki

Keigo Masaaki was the CEO of Psytech Corporation and one of the World's Leading Psysicists. He found another Ultraman statue in Kumamoto, similar to the ones that Tiga was imprisoned in and would quickly bond with it, becoming the being known as Evil Tiga.

History
According to Horii of GUTS, Masaaki was a genius even during his High School days and he took his intellect to create TPC and The Stellar Development Department.

Once he had learned of Daigo's assimilation with Ultraman Tiga, As well as the fact that they both bear a similar DNA to Tiga, Masaaki felt Daigo does not deserve the monopoly he has on the light. Instead he believed he himself deserves the right to inherit the light and transcend his humanity to become god-like for his own selfish and destructive intent. To prepare himself, Masaaki made an alliance with Professor Tango of TPC (a Scientist who felt he was not given proper credit for the work he had done.) and Keigo himself trained his body to be strong and powerful, and physically beat an exhausted Daigo (the latter of whom just defeated Geo Shark in a trap that was setup by Masaaki himself,) and thus stole the Spark Lens from him. After Daigo was lead to Masaaki's base in a cavern by a stray dog, Daigo watched in horror as Masaaki placed the stolen Spark Lens in a converter, changing his body into light particles and merging with the Ultraman statue. All while Daigo was unable to stop him as the convertter was covered by an electric barrier.

However, Keigo's greedy and selfish intentions corrupted the new giant of light into a giant of darkness known as "Evil Tiga," causing him to start rampaging into the city with his uncontrollable power. Ultimately, Daigo was able to recover the Spark Lens as he forces himself to endure the electricity and is able to break through by force of will to the Spark Lens. After a bitter fight, Tiga gains the upper hand and wounds Evil Tiga enough that he can fire his Zepellion Beam, vaporising his corrupted counterpart. While sharing most of Tiga's powers, Evil Tiga also shared his weakness of only being able to remain on Earth for a certain period of time. With Evil Tiga vanquished, Keigo was captured and imprisoned by authorities.

In the end of the series, Keigo returned to help GUTS revive Ultraman Tiga after his defeat at Gatanozoa's hands, in order to redem his misdeeds. While GUTS kept Gatanozoa busy, Keigo went down in a submarine and used a beam of light in an attempt to revive the hero and while Gatanozoa stopped him, Keigo's attempt revived Daigo deep inside the statue, allowing him to see the children of the world call out to him and use their light to revive Tiga as Glitter Tiga.