Kase is a fellow survivor of Ventara, like Len, and has the ability to transform into Kamen Rider Siren, the sole female Kamen Rider, with a swan Advent Deck. She is an eager hunter who acts the complete opposite of Len, who was unaware that she had survived.
- In the Japanese dub of Kamen Rider Dragon Knight, Kase is voiced by Miyuki Sawashiro, who voiced Kiva-la in Kamen Rider Decade.
- Kase and Len were the only Ventaran Kamen Riders' whose Earth twins were never seen; Len was never vented and thus his Advent Deck was never given to anyone else, while Kase's deck was temporarily reassigned to Maya Young while she was trapped in the Advent Void.
- She Alludes To Mai Valentine From Yu-Gi-Oh