Designation: Kamen Rider Siren
Weapons: Blanc Visor, Wind Shield, Wing Lancer
Advent Deck: Swan
Advent Cards: Blancwing, Sword Vent, Guard Vent, Final Vent
Contract Beast: Blancwing
As the sole female Kamen Rider, the blonde and beautiful Kase holds her own with her contract beast Blancwing, a giant mechanical swan. Other than Len, she is the only other original Ventara Rider. Both Len and Xaviax thought she was dead as she was last seen removing Len from his stasis pod, their pods were last in the line of the Ventara Riders. She appears on Earth after Albert Cho was vented by Kit. Kase saves Kit from Danny Cho, calling him an impostor and demanding him to take his armor off. Danny retreats and Kase goes on to fight Kit, believing he is on Xaviax's side, until Len stops her. Siren proposes a test to prove whether Kit is a real Kamen Rider and he looses. She thinks Kit is a sweet kid but not sure about his warrior status. She and Len both love each other. Kase also has healing abilities, demonstrated on Kit when his knee is injured. She was vented by Kamen Rider Strike, cementing Len's vendetta against James. When Eubulon was freed by Len, he reassigned the Siren deck to Maya. He later rescued Kase from the Advent Void and she was injured and took bed rest. She was weary of Adam's excuse for betraying the Riders but convinced Kit to give him the benefit of the doubt. Maya later returns the deck to Kase, saying she rather expose the truth with her words. Her card reader, the Blanc Visor takes the form of a rapier and can conjure her Wing Lancer. She helped defeat Xaviax and was a team with Kit and Len, bringing back the Ventarans from Karsh.