He later met Ichijo at the police station to learn the archeologists were all slaughtered by "Unidentified Lifeform 0". However, as they leave, the two encounter a Grongi attacking the station, Yuusuke puts on Arcle to assume Kuuga's Growing Form and saves Ichijo from the Grongi. Yuusuke goes against Ichijo's wishes to help fight Zu-Gooma-Gu, only to be beaten by the stronger monster. But after seeing the daughter of the late Professor Natsume cry over her father's death, Yuusuke's resolve to fight for the sake to keep people from suffering him to assume Mighty Form, taking name of 'Kuuga' as he saves Ichijo from Gooma and destroys Gumun. Starting with inspector Kaoru Ichijo, Yuusuke eventually gains the trust of the police force by receiving the TryChaser 2000 from Ichijo.
While brought to Dr. Shuichi Tsubaki for a confidential check-up, Yuusuke learns that the Arcle's fusion had spread into his muscular and nervous systems. During his first battle with Zu-Badzu-Ba, Kuuga assumes Dragon Form and eventually masters it. Then while fighting Me-Bajisu-Ba, Kuuga assumes Pegasus Form. Bajisu attacks and Yuusuke is able to effectively use Pegasus Form's power to kill it. While dealing with Me-Giiga-Gi, Yuusuke is able to become Titan Form to defeat it. When poisoned by Me-Ginoga-De, Yuusuke manages to survive due to the Amadam with shock therapy. After training to bring out the "golden power" with Enokida's help, Yuusuke's battle against the promoted Go-Garima-Ba leads to an upgraded version of Kuuga's Titan Form, Rising Titan Form. The next two fights allow Yuusuke to become bring Rising Pegasus Form and Rising Dragon Form.
After healing from being wounded by Go-Gamego-Re, Yuusuke meets Chono as he took up being an artist since their last encounter before he finds the Grongi and kills him once achieving Rising Mighty Form, accidentally causing mass devastation. Because of the incident, the BeatChaser 2000 wouldn't to be given to Kuuga as the Go-Bada-Ba finally begins to play, saving Kuuga. Yuusuke is aided by Sugita in tracking down Bada until Ichijo arrives with the Beat Chaser which allows Kuuga to destroy Bada. However, the incident with the sadistic Go-Jaraji-Da, influences Yuusuke's behavior for the worse, causing him to subconsciously assume Kuuga's Ultimate Form while killing the Grongi. Later, Yuusuke learns the "Kuuga" character not of Linto origin, but is actually an incomplete Grongi character and may be tied to Number 0, later revealed as the Grongi lord N-Daguva-Zeba when he arrived to slaughter Gooma before making his way to Tokyo.
During a flawed attempt to get Go-Jaza-Gi, Kuuga senses Daguva and got wounded while his guard was down before he finally kills Jaza. After a post-discussion of the recent Grongi/Kuuga connections after the death of Go-Baberu-Da, Yuusuke makes his way to the 12 Arikawa Group Flute Competition, having brush in with Daguva before arriving to the Arikawa Group building after a hostage situation is resolved with Mika traumatized by the incident. As he escorts Mika to her train for home, Yuusuke tells her not to run away even if things get scary. During the battle with the last of the Go Grongi, Go-Gadoru-Ba, Kuuga is overwhelmed by the Grongi's ability to assume forms similar to his own. Severely wounded as a result, Yuusuke is brought to the hospital where after a question of ethics, Tsubaki agrees to induce an electro-therapy charge after his heart gave out. By the time he leaves the hospital, Yuusuke arrives to the stadium where Gadoru is fighting Ra-Dorudo-Gu. However in the ensuing rematch, Kuuga assumes the dark-armored Kuuga Amazing Mighty Form to kill Gadoru.
With all the Grongi warriors dead, Daguva finally reveals himself, overpowering Kuuga's Amazing Mighty Form and cracking the Amadam while killing numerous people. Knowing that he has no choice but to break his word and assume that form, Yuusuke visits to his friends one-by-one, knowing that he may not come back, with Sakurako realizing his intentions. With Ichijo joining him, Yuusuke tells him to shoot him in the Arcle should he go berserk and becoming the entity that bring ultimate darkness. Once assuming Ultimate Form, powered by his gentle nature rather than hatred, Kuuga battles Daguva, continue to battle even when both revert to human form and nearly getting himself killed. However, Daguva dies from his wounds and Yuusuke survives. Three months after the final battle, Yuusuke had begun a new adventure to travel the world, starting at a Cuba beach where helps some kids before walking off, continuing his adventure.