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.
On January 27, it was announced that Kasai Yuumi had tested positive for COVID-19 by PCR test. As a result, the January 29 and 30 shows of the Hello! Project 2022 Winter ~LOVE & PEACE~ tour was cancelled, and the Tsubaki Factory Kishimoto Yumeno×Akiyama Mao×Fukuda Marine FC Event 2022 ~Chou Chou Chou Nen2~ in Osaka was also cancelled.
On August 2, she will celebrate her 19th birthday at a fanclub event titled Tsubaki Factory Kasai Yuumi Birthday Event 2022, which featured two shows at LANDMARK HALL.
She has a cat named Mimi (みみ). She calls her Mimi-chan (みみちゃん).
She graduated from high school in March 2022.
The following list are notable friendships Kasai Yuumi has acquired:
Here is a list of nicknames that was used to refer to Kasai Yuumi:
- Yuumi (ゆうみ)
- Yuttii (ゆってぃー)
- Yuu-chan (ゆーちゃん)
- Name: Kasai Yuumi (河西結心)
- Nicknames: Yuumi (ゆうみ), Yuttii (ゆってぃー), 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:
- 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)
- [2021–] Ikuze! Tsubaki Factory (行くぜ！つばきファクトリー)
- 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.
- 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.