However, Eiji managed to force Ankh into promising him that he can transform to save lives and not just to gather Cell Medals. It was after facing the Greeed Kazari that Eiji is approached by the Kougami Foundation, offered a partnership with them in the fight against the Greeed. After Eiji inserts three Core Medals into the OOO Driver belt, scans them with the O Scanner, and then holds the O Scanner up to his chest, he transforms into Kamen Rider OOO. Kamen Rider OOO has multiple combinations of powers he can access that are referred to as Combos. Kousei Kougami tells him that he is actually the second to be Kamen Rider OOO. Ankh states that only Eiji can be the current Kamen Rider OOO as he was the one who undid the seal of the OOO Driver. Soon after facing the Greeed Kazari that Eiji is approached by the Kougami Foundation, and is offered a partnership with them in the fight against the Greeed. In the process, joined by Akira Date, Eiji learns more of the Greeed's desire to become whole and the reason why Ankh is unable to assume his complete form when Ankh appears.
However, things change when Eiji becomes a host to half of a set of purple Core Medals that react to his lack of selfish desires, the same presumed reason why he is able to use Kamen Rider OOO's powers without any sign of going berserk so far. Though Eiji managed to keep the purple Core Medals under control and with time, even use their powers at will, they are later revealed to be slowly turning him into a Greeed, evident from the fact that he has lost his sense of taste by the time Ankh is absorbed by his other self and Shingo is now well enough to survive on his own. However, Eiji's condition worsens when the freed Ankh, with three of his Core Medals utterly destroyed with Ankh's destruction, forcefully possessed Shingo's body again and turned his back on Eiji. Soon after, Eiji learns from Kousei Kougami that the only way to counteract the purple Core Medals is to acquire a desire of his own, something he is yet capable of doing as he turns into the dinosaur-like Eiji Greeed after absorbing two more purple Core Medals forcefully inserted by Maki to have him go on a rampage.
However, it is by then that Eiji remembers that his desire is to have power, although again it is so that he can help others. He later agrees to Kougami's suggestion to absorb all of the Cell Medals in the Kougami Foundation's possession to achieve that desire. However, it does not halt Eiji's Greedification, as he sees the purple Cores as part of his powers and refuse to hand them over to Kougami, continue using them even though they consume him. During his final battle, he uses all the Cell Medals in his body in a risky attack that fails to destroy Maki. But before Eiji rampages as a Greeed, Ankh gives Eiji his last three Core Medals, disappearing and allowing Eiji to assume Tajadol Combo to defeat Maki, who's destruction create's a black hole destoring Eiji's purple medals and sucking the other Core Medals into it , except Ankh's broken Taka Core. As Eiji falls back to the earth, Ankh appears to him, thanking him and saying his farewells, before disappearing into half of the broken Taka Medal that once held his consciousness. Saved by Kamen Rider Birth, he realized that the power that he seeks are the people that he had bonded with, and that he does not have to save people alone.
He received the other half of Ankh's Core Medal that Hina found and later leaves Japan and resumes his journey around the world, unaware that a spectral Ankh arm is following him. In Movie War Mega Max, it is revealed that Eiji is currently working together with Kougami Foundation on their attempt to create new Core Medals in hope of restoring Ankh's Core Medal. As a result, Eiji is helped by Ankh from the future and regains most of his Core Medals while obtaining the Super TaToBa Medals. In Super Hero Taisen, Eiji is revealed to have restored the purple Dinosaur Medals. Helping the Gokaigers, he gives them each a Core Medal from his set that become special Ranger Keys they use to transform the six main combos of Kamen Rider OOO.