- Name: Kasai Yuumi (河西結心)
- Birthdate: July 30, 2003
- Birthplace: Yamanashi, Japan
- Blood Type: O
- Hobbies: Dancing, cheerleading, sleeping
- Looks Up To: Suzuki Airi, Asakura Kiki
- Hello! Project groups:
- Tsubaki Factory (2021–)
Kasai Yuumi was announced to be a winner of the Hello! Project "Juice=Juice" "Tsubaki Factory" Goudou Shin Member Audition in a surprise reveal during episode #383 of Hello! Project Station. She would join Tsubaki Factory alongside fellow audition winners Yagi Shiori and Fukuda Marine, as well as Hello Pro Kenshuusei member Yofu Runo. Their activities with the group began at the Hello! Project 2021 Summer Sapphire & Ruby concerts starting July 17. At some point prior to auditioning she was signed with management company Amuse as an actress.
- TV Dramas
-  Kaitou Sentai Lupinranger VS Keisatsu Sentai Patranger (Ep. 12)
- TV Programs
-  Ikuze! Tsubaki Factory (行くぜ!つばきファクトリー)
- [2020–] Hello! Station (ハロ！ステ)
- Has experience in cheerleading.
- In her introduction she said she wants to do a Christmas gift exchange with the Tsubaki Factory members.
- Wants to be able to perform cutely.
- Her favorite type of music is J-Pop.
- Her favorite sport is field hockey.
- Her motto is "Always rise after a fall" (七転び八起き; Nanakorobi Yaoki).
- Her favorite foods are hard peaches, ramen, and hamburger steak.