The Sonozaki Family is an aristocratic family in Futo part of the organization known as the Museum. When his son Raito (Philip) died and was reborn as an avatar of the true Gaia Memory, Ryubee used his organization to produce and distribute the Gaia Memories in the criminal underworld, resulting in the Dopant crimewaves. This enabled the Museum to follow through with the Gaia Impact. With the exception of Raito and Fumine, each member of the Sonozaki Family owns a special Gaia Memory, called Gold Memories, allowing them to assume Dopant forms with the use of a prototype Gaia Driver belt which prevents the Gaia Memories from corrupting their minds. In the end, when Ryubee's plan almost succeeds, it results with the family estate burning to the ground, marking the end of both its patriarch and the Museum. At the end of the series, when Wakana sacrifices her body to give Philip (Raito) his body back, the family becomes a part of the Earth to watch over Philip and tasks him with the family duty to protect Futo and to change the world for the benefit of mankind.
Ryubee Sonozaki is the head of the Sonozaki Family and leader of the Museum, acting as a godfather in the organization and seeing the entirety of the city of Futo as his personal kingdom, not letting anyone get in the way of his vision for the city, not even his wife and children. With the Terror Memory, Ryubee can transform into the Terror Dopant, who can increase the fear in his victims to maddening levels. He could also summon the Terror Field to burn his victims or teleport himself and others across long distances and project his power into the Terror Crown to create a Barong-like creature familiar, the Terror Dragon. As the head of the Museum, Ryubee oversees the distribution of Gaia Memories, using those who use them as research specimens, regardless of the harm and deaths that ensued. He does not seem to consider the Kamen Riders fighting the Dopant crime wave a threat, using the data off of them as a means to perfect his plans for Wakana before turning his attention to regain Philip and complete his plan to initiate the Gaia Impact.
It is revealed that Ryubee was an archaeologist who bought the grounds his estate was built on after finding deposits of fossils and relics beneath it, forming the collection at the Futo Museum where he works as its curator. He eventually discovered the location of the planet's consciousness and brought his family to see it. However, when his son Raito fell into the "fountain" of the Gaia Memory and died, but was somehow resurrected by the planet's powers, Ryubee began his plan for the Gaia Impact: an event that would ensure the future of the human race by making them one with the planet itself, much like his son had become. To reach his goal, he used an item dubbed the "Evil Tail" , an archaeologist's brush with the names of his family inscribed on it, to overcome the fear that would go along with sacrificing them to reach his goal, eventually making him go mad. When he sacrifices Raito (Philip) once more into the Gaia Memory to make his daughter Wakana one with the Earth, he is confronted by both Kamen Riders. While Kamen Rider Accel destroys his Terror Dragon, Shotaro is able to retrieve Philip's mind and then his body from the aspects of the Gaia Memory in ClayDoll Xtreme to transform into Kamen Rider Double CycloneJokerXtreme to defeat the Terror Dopant and destroy the Terror Memory, along with most of the Sonozaki estate. With Ryubee's mind still broken from years of planning, he enters the still burning ruins of his home, glad that he finally initiated the Gaia Impact. He dies in the blaze, thinking of the good times he had with his family before he discovered the Gaia Memory.
Saeko Sonozaki is the eldest of the Sonozaki children who runs an IT company called the Digal Corporation, which serves as a front for Gaia Memory production and distribution. As the Taboo Dopant, she gains the ability to fly and generate deadly bombs of intensely hot light. Ryubee strictly raised Saeko to believe that their family is superior to all others and that they are destined to rule the world. This loveless upbringing caused her to grow up secretly hating her father, often taking the resulting rage out on her younger sister Wakana, serving as fuel for her own ulterior motives of taking over the Museum to prove herself its ideal successor. 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. After Kirihiko's death, Saeko begins to visit Shinkuro Isaka to improve her Dopant form and focus on her plans for the Museum. After he is almost defeated by the Kamen Riders, she begins to show that she cares for Isaka's well-being. Once her coup against her father fails and Isaka is killed off, Saeko goes on the lam and is hunted down by the Smilodon Dopant, who takes the Taboo Memory and leaves her for dead. However, Saeko is saved by Kazu who offers his support in her scheme to take control of the Museum. After infiltrating the Digal Corporation to obtain the Nasca Memory and then using an L.C.O.G on herself, Saeko uses the Gaia Memory in its Level 3 state to become the R Nasca Dopant ("R" for both "Red" and "Revenge") to overpower her sister. However, when Wakana evolves to ClayDoll Xtreme, Saeko is forced to go after other options to make herself more powerful, such as the Jewel Memory, before the Nasca Memory is destroyed in a confrontation with ClayDoll Xtreme. Later, after her father's death, Saeko is disillusioned until she learns of Wakana's survival as regains the Taboo Memory from Kazu after he announces that she has become the head of the Museum. However, no longer having a reason to prove herself, Saeko fights Kazu in order to save her sister. As a result, the Taboo Dopant sacrifices herself to hold off the Utopia Dopant to give Wakana time to run away, resulting in Saeko's death as she finally admits the irony of her last moments in her final breath.
Wakana Sonozaki is the youngest daughter and middle child of the Sonozaki family who since her brother's death has been secretly raised to become the key element in her father's plans. Working as a DJ at the local Wind Wave radio station, she hosted the Wakana's Healing Princess show and gained a fanbase as a result. While she puts up a shy and kindly persona on the show, her true personality is the complete opposite. As a member of the Museum, Wakana is able to transform into the ClayDoll Dopant , enabling her to produce gravitational energy shots, as well as the ability to reform her Dopant body after it is shattered into thousands of pieces. Out of all of the members of the Sonozaki Family, she is the least involved with the Gaia Memory business, as she was doted upon while growing up. After encountering Philip, Wakana begins to have second thoughts about being a Dopant and discards the ClayDoll Memory. It is returned to her by her father, who is aware that she did not lose it as she claims; he later decides to bring her into the family business after Kirihiko's death and is the one chosen to view the effects of the Xtreme Memory on the true Gaia Memory. Before this, Wakana also becomes one of Shinkuro Isaka's unwilling experiments after he modifies her Gaia Driver, subjecting her to more of her Gaia Memory's influence and its short term side effects showing her compatibility with its powers.
After trying to run away with Philip, her father takes her to the Museum's true base of operations. Believing that she is serving a greater good for the planet's sake, Wakana runs the Museum and the Digal Corporation, bent on bringing Philip back to the Museum to complete the Gaia Impact. She is also given the Gaia Progressor, designed to evolve her into the godlike ClayDoll Xtreme to prepare her for the Gaia Impact so she become one with the planet's vast knowledge. While armed with tendrils and exponentially more power than either the ClayDoll Dopant or CycloneJokerXtreme, she becomes a being like Philip with access to the true Gaia Memory. When the Gaia Impact seems to succeed after Philip is thrown into the source of the true Gaia Memory, becoming a living Gaia Memory and fully integrated with the ClayDoll Memory, Wakana loses control when Philip is removed from her body, causing an explosion that destroys the Sonozaki estate. However, in midsts of the blaze, Wakana's unconscious body is carried off by Kazu to the Foundation X research facility to be of use in his own version of the Gaia Impact. However, Shotaro manages to save her before taking her to the hospital. A few days later, Wakana attempts to force her way out of the hospital to resume her family's goal before finally learning that Philip disappeared into the Gaia Memory. After getting the means to invoke the Gaia Impact from Shroud in her final moments, Wakana sacrifices her own existence to bring her brother back via the Xtreme Memory.
Kirihiko Sudo is a young man who initially works for the Museum as one of its best Gaia Memory dealers. He also has a professed love of the city of Futo, and in his youth created the city's mascot Futo-kun. In his work with the Museum, Kirihiko eventually catches the eye of Saeko and they marry, taking on the Sonozaki surname in place of his own. In being welcomed into the family, he is granted a Gaia Driver along with the ability to become the Nasca Dopant, an expert swordsman armed with the Nasca Blade with the powers of flight from the Nasca Wings. As an in-law, Kirihiko is not as knowledge in the truth about Gaia Memories or Kamen Rider Double's existence until confronting him in battle. He eventually discovers that Shotaro is Double, and begins to confront him as a rival to test his Gaia Memory's powers. When he finally discovers the truth behind the Gaia Memories, while suffering near-lethal side effects from eventually reaching Level Two of the Nasca Memory's powers, he tries to take Saeko away from the Museum, but she kills him as a threat to her future plans. Saeko kept his Nasca Memory, eventually using it to transform into the R Nasca Dopant.
Mick is Ryubee's beloved pet blue British Shorthair, named by his son Raito (Philip). Despite being a cat, the Smilodon Memory can transform Mick into the humanoid Smilodon Dopant that can can discharge electricity and move fast enough to evade the homing shots of Kamen Rider Double LunaTrigger and outmaneuver Kamen Rider Accel Trial. He appears to be more intelligent than the average house cat, in that he appears to know that he is able to transform into the Smilodon Dopant and he knows how to transform back into a house cat at will. He is often utilized by the Sonozaki Family to deal with matters outside their estate, such as hunting down Philip and traitors, as well as bodyguard and spy. Mick also appears to favor Wakana over the other members of the family though will faithfully follow Ryubee's orders. While targeting Kyoko, the Smilodon Dopant defeats Accel Trial before Double CycloneMetal catches the cat off guard before destroying his Gaia Driver and Smilodon Memory with the Metal Stag Break Maximum Drive. After Ryubee's death, Mick is brought to live at the Narumi Detective Agency.
"Shroud" is a mysterious woman who is seeking revenge against Ryubee Sonozaki and the Museum. Ryubee refers to her as the embodiment of rage. Her face is wrapped in bandages to obscure her identity, and she is always seen wearing sunglasses, a large black fedora, and a black trenchcoat. Shroud first appears to provide Ryu Terui with the equipment he needs to be Kamen Rider Accel. She later sends blueprints and Gaia Memories to Philip to build the Frog Pod and Denden Sensor. When Philip is nearly killed by the Weather Dopant, Shroud approaches Philip via the Xtreme Memory, addressing him as Raito, to convince him to end his partnership with Shotaro as he is only holding back his powers. Shroud also gives Ryu the Trial Memory to train him to become Accel Trial, planning to have him die due to his inability to use the Gaia Memory properly and his change of heart in his anger towards Isaka.
After it is revealed to Ryu that Shroud is the one who gave Isaka the Weather Memory, she reveals that she is Fumine Sonozaki, wife of Ryubee and mother to Saeko, Wakana, and Raito (Philip). While a member of the Museum, she developed the Kamen Riders and T2 Gaia Memories. After Raito died and was revived by the Gaia Memory, she wished to take him away from her husband who only wanted to use the boy as a tool in his plans. After he attacked her as the Terror Dopant, severely scarred by his Terror Field in the process, she left him and spent the next 10 years plotting her revenge. She claimed that her first attempt was to use Isaka to kill her husband, but she didn't predict his murderous streak. She later provided Sokichi Narumi with the needs to build the hangar beneath his agency's offices, as well as the Lost Driver and Skull Memory; only if he would be able to save Raito from the Museum. However, after seeing that her plans on having Terui and Philip transform together into CycloneAccelXtreme would be unnecessary, she decides that she no longer needs to fight for revenge, and allows the Kamen Riders to fight as they wish. She returns to the family estate one last time, after Philip is taken in to allow Wakana to initiate the Gaia Impact, and imparts with him the knowledge that Shotaro is a wild card; this allows him to later save himself from the Gaia Memory, thwarting the Museum's plans. After the final battle with Foundation X, Shroud is seen in a flashback where she speaks with Wakana who seeks to initiate the Gaia Impact on her own. After telling her daughter the info she needs is in the Gaia Library, Shroud dies.
Shroud has her own refined Gaia Memory called the Bomb Memory which she uses in the Shroud Magnum. Shroud also has several supernatural powers that don't have a basis in Gaia Memory use. She is able to summon up rings of fire that produce various objects, as well as disappear and reappear out of thin air.