Ryubee Sonozaki is the head of the Sonozaki family and the Museum organization, the principal antagonist of the Kamen Rider Double series.
Ryubee and his wife were responsible for creating the devices called Gaia Memories, which allow a person to transform into a monster (called a Dopant) by accessing the memories of the planet. The Gaia Memories are addictive, and prolonged use can cause mental damage. They also created the Double Driver and Lost Driver, which allow a user to transform with Gaia Memories, without the risk of addiction or bodily harm. These people were referred to as Kamen Riders, such as Kamen Rider Double, Kamen Rider Skull, and Kamen Rider Eternal.
Ryubee uses the Terror Memory, and a special Driver, to become the Terror Dopant.
His wife, Fumine, left him after the death of their son, Raito. He carried on his research, and distributed Gaia Memories in secret to the people of Fuuto City, to observe their effects on the human body. Each of these Dopants was defeated by Kamen Rider Double, and later Kamen Rider Accel.
After adequate research from his Dopants, and observing Kamen Rider Double, he chose his daughter Wakana, to become one with the Earth's knowledge by way of an event called the Gaia Impact. Before he can do this, he is defeated by Kamen Rider Double and Kamen Rider Accel, and dies as his home burns around him.