"I'll evolve and transcend over this weak human existence (laughs). Ancient power, transform me into the giant of light!"
- ―Keigo Masaki gloating over his (forced) assimilation with Evil Tiga.
Keigo Masaki (マサキ・ケイゴ Masaki Keigo) is the CEO of Cytech Corporation (サイテックコーポレーション Saitekku Kōporēshon) and one of the world's leading physicists. He found another Ultraman statue in Kumamoto, similar to the ones that were found in Tiga's Pyramid and desired to bond with it, eventually becoming the being known as Evil Tiga.
According to Horii of GUTS, Masaki was a genius even during his high school days and he took his intellect to start the leading space development company.
Once he had learned of Daigo's assimilation with Ultraman Tiga, as well as the fact that they both bore similar DNA to Tiga, Masaki felt Daigo did not deserve the perceived monopoly he had on the power of 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 greedy intent. Masaki felt that it was the duty of the Ultramen to guide man's evolution, in effect control them, where as the Ultramen had always only served mankind as their protectors. For this reason he felt Daigo was wasting his time with trivial matters and believed he only helped people for his own satisfaction of being a hero. To prepare himself, Masaki 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 Geozark in a trap that was setup by Masaki himself,) and thus stole the Sparklence from him. After Daigo was led to Masaki's base in a cavern by a stray dog, Daigo watched in horror as Masaki placed the stolen Sparklence in a converter, changing his body into light particles and merging with the Ultraman statue. All the while Daigo was unable to stop him as the converter 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. Soon another creature arose to face him the Ultra-canine Guardie, who had come to stop his corrupted master. Masaki trounced Guardie. GUTS blasted him to stop, but he blasted the jets. Ultimately, Daigo was able to recover the Sparklence as he forced himself to endure the electricity and was able to break through by force of will to the Sparklence. When Tiga arrived, Evil Tiga murdered Guardie, vexing the warrior of light with that act. After a bitter fight, Tiga gained the upper hand and wounded Evil Tiga enough that he could fire his Zeperion Beam, vaporizing 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.
At the end of the series, Keigo was recruited by GUTS to help them revive Ultraman Tiga after his defeat at Gatanothor's hands. While GUTS kept Gatanothor busy, Keigo went down in the Dolphar 202 and used the Maxima Converter Unit (マキシマ・コンバーターユニット Makishima Konbātā Yunitto) (similar to the one he used to become Evil Tiga) in an attempt to revive the hero and while Gatanothor 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, thus allowing Keigo to redeem himself for his misdeeds.
- Height: 178 cm
- Weight: 62 kg
- Blood Type: B
- Birthplace: Kagoshima Prefecture, Japan
- Birthdate: July 31, 1982
- Powers and Weapons
- Intelligence: As stated by Horii, Keigo was a child genius and one of, if not the brightest scientists on Earth, using his knowledge he created Geozark, a mechanical construct that was as powerful as the monsters GUTS usually fought. He was also able to hack into GUTS communication and remotely take over one of their fighters. Later his knowledge of Ultraman mechanics allowed Daigo to harness the light of the worlds' youth to fight the king of darkness.
- Conditioning: Due to his obsession with gaining Ultra powers, Keigo honed his body to its limits to prepare himself for his self deluded role, he was able to easily trounce a tired Daigo.
- Transformation: As he bears Ultraman DNA like Daigo, in theory, Keigo should also possess the ability to transform into living light under danger, it is unknown if he would transform into Evil Tiga as Daigo transforms into Tiga.
- At the end of the series, Keigo says that he also has had the same dream about the previous destruction of mankind, thanks to bearing the same DNA as Daigo.