Played by Kamio Yuu 神尾佑
The main villain in Kamen Rider OOO.
Maki Kiyoto is the brilliant mind who designed the Medal System for Kougami along with the Birth Driver and its equipment. His parents died an early age and was raised by his much older sister, Maki Hitomi. He was neglected by his sister to the point that she suggested a doll as a replacement for her. One night before her wedding day, Hitomi was killed by Maki when he burned her alive. At that point on, he remembers a different version of her sister being kind not the one who neglected him. His history between killing his sister and to the point where he became the developer of the Medal System is unknown.
When Maki first appears in the TV series, he would show up at various crime scenes since he is collecting data on the Greeed and Yummy due to discovering one of his workers was making bombs and became a parent for the Yummy. When Kazari discovers that one of his Medals was discovered by someone else, he and Maki entered a partnership which Maki would provide ideas to advance his agenda of finding a container for the Medals that would go out of control.
When Date brings a flyer of Cous Coussier back with him, Maki notices that Chiyoko is similar to his dead sister. This became an obsession to the point that he was found out by Kougami for working with Kazari. At the same time, Kazari became frustated with Maki unable to choose a side and creates the Shachi-Panda Yummy from his desire for Hitomi to love him. He tries to fight off the Yummy but gets injured during the encounter which Chiyoko appeared. She took him on his request to his old home. While tending to Maki's wounds, Chiyoko finds a photo of his sister and realizes that she looks like her. She asks Maki about his sister which he remembers how his sister is really like along with him causing the fire. Chiyoko's kindness reaffirms Maki's true goal and defects from Kougami to Kazari's side.
Maki blew up most of his lab at the Kougami Foundation along with a bunch of Candroids and his painting but took one more thing as well. He took a sealed container that contain 10 purple Medals that Kougami found in his recent trip to Europe. He opened the container and removed one of the purple Medals from it. He absorbed five of the purple Medals and started the process of becoming a Greeed himself. He needed the other five purple Medals from Eiji so he could defeat the Greeed who became out of control from having the Core Medals inside of them. Maki soon begins to take the Core Medals of the fallen Greeed like Kazari, Mezool, and Gamel. At one point, he agreed for Ankh to be the container for the Medals since he lost three of his Core Medals but ends up changing his mind since Ankh was having second doubts about it. He tries to get Eiji's purple Medals by injecting two more Medals into him to speed up his own Greeedification but fails due to Eiji wanting the power of the purple Medals. In the end, Maki uses Uva as the container since he was the only Greeed that was fully alive and had a small container for desire. Maki leaves his doll to Chiyoko before engaging in the final battle between Eiji and Ankh. He put up a good fight even though only had three purple Medals but was defeated by Eiji in the end. Maki became a black hole and sucked up all the remaining Core Medals except for Ankh's cracked Taka Core Medal.
Maki is a cold person who cares about the end of something. This line of thought came from his sister, Hitomi, who told him that the end completes a person. He is the complete opposite of Kougami who indulges in the birth of things. Maki wanted the world to end before it becomes ugly like his sister did on the inside. He views people as things to complete his goals like trapping a bunch of Kougami Foundation employees to get data on the Sa-me Yummies. To a certain point, he even treated the Greeed like experiments and once their use was over, he got rid of them.
Maki has a very interesting quirk that involves his doll, Kiyo-chan. It was given by his sister as a replacement for her but lost its hair in the fire. He has the doll on his left arm and usually talks to the doll even when speaking to other people. When his doll gets knocked off his arm or loses control of it, he begins to make weird noises and his voice changes from a low pitch to a very high pitch. He ignores everyone else and focused on the condition of his doll in this state.
- Maki's doll is named Kiyo-chan which is similar to his first name.