Marina Tsumura is Yousuke's girlfriend and also Shotaro Hidari's childhood friend.
Same as her boyfriend, she was also fired from her position as Windscale's top designer, buys the T-Rex Memory with her boyfriend Yousuke to exact their revenge for both being fired. However, when Yousuke goes berserk, Marina hires her childhood friend Shotaro to find Yousuke so she can kill him. Once Double destroys the Magma Memory, Marina uses her Dopant form to abduct and murder Yousuke. After Shotaro learns the truth, Marina tries to kill him as she assumes the T-Rex Dopant form. After a brief fight, T-Rex Dopant then transforms into the Big T-Rex and leads Double on a chase around the city. As Kamen Rider Double, Shotaro destroys the T-Rex Memory with HeatMetal's Metal Branding Maximum Drive and hands Marina over to the authorities.
- As the T-Rex Dopant, she can cause massive shockwaves with its roars.
- To assume this Big T-Rex form, she must use debris to form a robotic T. rex body to increase its powers while in default T-Rex Dopant form.