Kasai Yuumi was born on July 30, 2003 in Yamanashi, Japan.
On July 7, Kasai was announced as one of the winners of the Hello! Project "Juice=Juice" "Tsubaki Factory" Goudou Shin Member Audition audition and would join Tsubaki Factory alongside Yagi Shiori, Fukuda Marine, and Yofu Runo.
As of April 2021, Kasai is currently in her last year of high school.
Here is a list of nicknames that was used to refer to Kasai Yuumi:
- Yuumi (ゆうみ)
- Yuttii (ゆってぃ)
- Yuu-chan (ゆーちゃん)
- Name: Kasai Yuumi (河西結心)
- Nicknames: Yuumi (ゆうみ), Yuttii (ゆってぃ), Yuu-chan (ゆーちゃん)
- Birthdate: July 30, 2003
- Birthplace: Yamanashi, Japan
- Blood Type: O
- Western Zodiac: Leo
- Eastern Zodiac: Goat
- Hello! Project Status:
- 2021-07-07: Tsubaki Factory member
- UP-FRONT PROMOTION Status:
- 2021-07-07: Member
- Tsubaki Factory Member Color: Purple
- Hello! Project Groups:
- Tsubaki Factory (2021-present)
-  Good Partner (Case 2)
-  Yakeni Ben no Tatsu Bengoshi ga Gakko de Hoeru (ep. 2-3)
-  Kaitou Sentai Lupinranger VS Keisatsu Sentai Patranger (ep 12)
- She is the first member in Hello! Project history to be from the Yamanashi Prefecture.
- She has experience in cheerleading.
- She wants to do a Christmas gift exchange with Tsubaki Factory members.
- She wants to be able to perform cutely.
- Before joining Tsubaki Factory, she was previously signed with management company Amuse as a child actress.
- She appeared as a minor character in episode 12 of TV Asahi's Kaitou Sentai Lupinranger VS Keisatsu Sentai Patranger, the 42nd Super Sentai series, which starred former Morning Musume member Kudo Haruka as Hayami Umika / Lupin Yellow.
- Her audition number was 246.
- Gallery:Kasai Yuumi
- List:Kasai Yuumi Discography Featured In
- List:Kasai Yuumi Concert & Event Appearances
- List:Kasai Yuumi Publications Featured In
- List:Kasai Yuumi Pocket Morning Weekly Q&A
- 河西 結心 - アミューズ オフィシャル ウェブサイト (in Japanese). Amuse Talent Official Website. (archived)
- "「Juice=Juice」「つばきファクトリー」 新メンバー決定！" (in Japanese). Hello! Project. 2021-07-07
- #12「魔法の腕輪」 (in Japanese) Kaitou Sentai Lupinranger VS Keisatsu Sentai Patrangerofficial website.