(from wikipedia)
Eiji Hino (火野 映司 Hino Eiji?, portrayed by Shu Watanabe) is the son of a politician who travels the world with his grandfather. While in Africa a year prior to the beginning of the series, Eiji attempted to use his fortune to help out a small village. However, his money was used to fund a civil war that cost the life of a little girl Eiji befriended, whose death he was unable stop and was forced to looked on helplessly. Though the Hino family used his sob story to get the people's support, the ordeal caused Eiji to lose all selfish desires and become willing to help those in need, going so far as to even risking his life for complete strangers.
Returning to Japan to fund more of his world travels, Eiji's latest part-time job at the Kougami Art Museum ends due to the destruction of the museum around him while in a drugged slumber, taking a strange red coin he found as his supposed pay. This causes a disembodied arm to begin to follow him around, demanding the red coin from him which is actually a part of his being. However when attacked by a Yummy, the arm revealed himself as Ankh of the ancient beings known as the Greeed, whose the red coin is one of his Core Medals. When Eiji risks his life to rescue him, Ankh gives him two more Core Medals and the OOO Driver as means to transform into Kamen Rider OOO. After destroying the Yummy, Eiji learns more about the Greeed and Medals for Ankh, who originally intended for Eiji to be a tool for gathering Cell Medals for him. 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. 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. 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, with dangerous consequences.