Saeko Sonozaki

Saeko Sonozaki is the eldest child of the Sonozaki Family and a major antagonist of Kamen Rider Double.

Background
After her father began his plans for the Gaia Impact and strictly raised Saeko to believe their family was superior to all others and that they are destined to rule the world, Saeko secretly hated her father for never showing her fatherly love. As a result she often took her rage out on her sister Wakana serving as fuel for her own ulterior motives of taking over the Museum. At some point she became the owner of the IT company, Digal Corporation.

Personality
Due to way Ryubee raised her, Saeko hated her father and planned to take control of the Museum from him, but still stuck to her duties for the organization. Her relationship with Wakana has changed over the series from a simple older sister abuse on Wakana to a full-fledged sibling rivalry for control of the Museum. Saeko does not seem to mind that her brother, Raito (now Philip) is now her enemy and has shown less interest in retrieving him than Wakana and Ryubee.

She also has a tendency to kill her boyfriends if they do not live up to her expectations. When Saeko spares Kirihiko, she marries him and lets him live until he threatens the Museum's livelihood and kills him. Saeko held a close relation with the Museum physician Shinkuro Isaka, which started as a way to improve her Dopant form but grew closer to the extent of caring for his well being more than his defeat from the Kamen Riders, unlike Kirihiko where she abused him for his failure. Saeko currently now is under the partnership of Foundation X researcher, Jun Kazu who possesses feelings for Wakana. But it is unknown how she feels about him.

Dopant forms
As a member of the Sonozaki Family, Saeko owned a Gaia Driver and a golden Gaia Memory, the Taboo Memory to become the Taboo Dopant. But when they are taken from her by the Smilodon Dopant, she uses her ex-husband's Gaia Memory in place of them until it is destroyed.

Taboo Dopant
The Taboo Dopant is Saeko's first Dopant form accessed through the Gaia Driver and Taboo Memory. In this form she has the appearance of a rag doll and gains the ability to fly and generate deadly bombs of intensely hot light. It was further increased in power due to some modifications to her Gaia Driver by Isaka, to the extent of being unaffected by certain Dopant powers such as that of the Yesterday Dopant. This form is lost after Mick as the Smilodon Dopant takes her equipment away and nearly kills her.

R Nasca Dopant
The R Nasca Dopant is Saeko's second Dopant form, unlike her previous Dopant form she used the L.C.O.G on herself to create a Living Connector to use the memory instead of a Gaia Driver which limits the powers of the memory. Unlike Kirihiko's form which is blue with red markings, Saeko's is an orangish-red and possesses greater speed and power due to being Level 3. The R in the name stands for both Red (the forms color) and Revenge (Saeko's feeling towards her father and sister). But when Wakana evolves into ClayDoll Xtreme Saeko went after other plans to empower herself further such as the Jewel Dopant. However the Nasca Memory was destroyed in a fight against ClayDoll Xtreme.

Part in Story
In the first part of the series Saeko is usually seen at Digal Corporation and working with Kirihiko, distributing the Gaia Memories and becoming the Taboo Dopant to combat Kamen Rider Double. Later when she kills Kirihiko and starts visiting Shinkuro Isaka to improve herself, she forms a partnership with him to overthrow her father and tries to kill Wakana for not taking her side but the Smilodon Dopant drives her away. The Weather Dopant is then defeated by Kamen Rider Accel Trial and her coup fails and ends up on the lam. She is then chased by the Masquerade Dopants before being finally attacked by the Smilodon Dopant and nearly killed losing her Taboo Memory and Gaia Driver in the process. She is rescued by Jun Kazu who offers her protection out of love but Saeko instead uses the Level 3 Nasca Memory instead to overthrow Wakana until going after other options to empower herself further after Wakana evolved into ClayDoll Xtreme. Saeko then visits her family's estate to have dinner with her family one last time for her and her estranged mother to apologize to Wakana before she becomes one with the Earth but Saeko refuses and instead fights her sister only for the Nasca Memory to be destroyed. The next day when Ryubee and the Sonozaki estate are destroyed Saeko is seen seeing it burn to the ground along with Shroud.