Saeko Sonozaki

Saeko Sonozaki is the eldest child of the Sonozaki Family as well as an executive of Museum and the secondary antagonist of Kamen Rider Double.

History
One year prior to the start of the show, Saeko killed Sokichi Narumi, the mentor of Shotaro Hidari. Saeko married Kirihiko Sudo, who worked for Museum as a Gaia Memory distributor. However, after Kirihiko found out about Museum's true goals and attempted to persuade Saeko to leave the organization she killed him.

Saeko met Shinkuro Isaka, the Weather Dopant, soon after and began dating him. Envious of her father's increased favoritism towards Wakana Sonozaki, Saeko begins plotting with Isaka to overthrow her father and seize control of Museum. However, after Isaka is defeated by Kamen Rider Accel, Saeko's plan is foiled and she is forced to go on the run. The Smilodon Dopant hunts her down, taking the Taboo Memory from her and leaving her for dead.

However, Saeko was saved by Jun Kazu, a researcher for Foundation X, who teams up with her to take control of the Museum. Saeko's first move for her second coup was to infiltrate the Digal Corporation, now run by Wakana, and steal the Nasca Memory. Saeko then goes to confront Wakana and, with a Living Connector, uses the Gaia Memory in its Level 3 state to become the R Nasca Dopant. While Saeko had the power advantage in the fight at first, after Wakana evolves to ClayDoll Xtreme, Saeko is swiftly overpowered by it and eventually loses the Nasca Memory after it is destroyed.

After her father's death, Kazu gives her back the Taboo Memory and tells her that she is the new head of the Museum. However, with her father gone and no one to prove herself to, Saeko becomes lost with her life. Saeko later discovers that Wakana is still alive and goes to save her from Kazu, who becomes the Utopia Dopant to attack Wakana and reawaken the Gaia Impact. Saeko becomes the Taboo Dopant and fights Kazu off in order to save Wakana. Unable to stand up to the Utopia Dopant's power, Saeko sacrifices herself to give her sister time to escape. As she dies, Saeko admits the irony of sacrificing herself to save someone whom she was envious of for so long before passing away.

Saeko later appeared with the rest of her family to say goodbye to Philip. she reconciled with Wakana whom she comforted.