Kase

A character from Kamen Rider: Dragon Knight, Kase is a Ventaran who transforms into Kamen Rider Siren and a love interest of Len.

During the time Adam "betrayed" the other Riders, she quickly took herself and Len out of suspended animation and managed to throw him into a mirror. She was thought to have been lost by both Len and Xaviax. Before escaping, she managed to steal two Survive Mode cards for Dragon Knight and Wing Knight (unable to get her own in time). She battles Xaviax's monsters in secret and slowly makes her way back to Len. She first appeared to fight off Xaviax's minion Mirror Monsters and easily defeats them.

She later crashes the battle between Kamen Rider Dragon Knight and Kamen Rider Axe (before the latter could use his Final Vent on the former). She battled Kamen Rider Axe, which ends with Kamen Rider Axe retreating. She then attacks Kamen Rider Dragon Knight until Kamen Rider Wing Knight appears and reveals that she knows him, when she asks why Len is "protecting Xaviax's Riders." It was revealed that Kase is Len's girlfriend, and they are overjoyed to see each other alive. She decides to test Kit by battling him, and ends up defeating him in a sparring match. She does not accept Kit as the new Dragon Knight and feels that Kit is too inexperienced to even be considered a Rider, but trusts Len's judgment. She feels uneasy the next time she sees him as she begins to see the similarities in personality that Kit and Adam has, stating that it was the attitude Kit has that started Adam's betrayal. But after knowing his resolve, she accepts Kit as a Kamen Rider and apologizes for her earlier behavior. While waiting for Kit to return, Len and Kase sense Strike and Wrath and manage to save Lacey and Trent, but they are easily overpowered by the Xaviax-powered Wrath and Strike. They are able to escape before sustaining any more injuries. Eventually, Trent gives them the location where Kit is being held and they attempt to rescue him. They find him being attacked by Wrath and Strike, but manage to save him. Returning to the bookstore, she heals Kit's injured leg. After escaping the No-Men from the bookstore, she shows Kit and Len their Survive Cards, saying they will be necessary as they are no longer fighting amateurs. Soon afterward, Kase is forced to battle both Wrath and a hybrid monster. When her back is turned, Strike uses his Final Vent, venting her. Kase is the eighth Kamen Rider to be vented in the series. Len retrieves his fallen girlfriend's deck and promises revenge for her. Her venting was avenged when Len, using Survive Mode, vented Strike. After Kase was vented, Eubulon transferred her Advent Deck to Maya Young. Kase is later rescued from the Advent Void by Master Eubulon, and reunites with Len while she is recovering. Afterwards, Maya returns the Advent Deck to her; Kase says that she is proud of her not just for her work as a Kamen Rider, but also as a journalist who exposes the truth amongst the lies. She later speaks with Kit in private and convinces him to give Adam a chance to prove himself like she did with him. After Maya senses something wrong with Adam, Kase and Kit go off to follow Len and Adam. They find the two fighting each other. She soon finds out from Adam that he had given Xaviax the virus who decoded it and created a vaccine. She forgives him as she realizes that he is needed in the final battle and that he truly is sorry for his previous actions, reaffirming this with Kit. Kase and Kit both distract Xaviax and his monsters from detecting Trent and Maya who both manage to get a mirror inside Xaviax's base. Kase and the others manage to get inside and together with Len, Kit and Adam, fight off Xaviax. Though no match for the mighty General, they fought him off long enough for Eubulon to rescue the other Riders and together, they all use their Link Vents to destroy Xaviax. During the Epilogue, she continues to date Len as she helps rebuild Ventara and protect Ventara and Earth from remnants of Xaviax's army as well as dangers from other worlds behind the mirrors.