Kei Asechi, a retired Prism Star turned legendary trainer, lives secluded in a remote European-style mansion in Japan. She once shared this home with her former husband Kintaro Asechi, their daughter Kyoko Asechi, and her later adoptive son, Jun Takigawa. Kei's demanding training methods, requiring trainees to discard emotions to master the Aurora Rising technique, are viewed as cruel. This approach sparked conflict with Prism Stars like her daughter Kyoko and Mion Takamine, though others such as Rizumu Amamiya and Sonata Hanami submitted to her regimen.

Her marriage to Kintaro ended in divorce due to his dangerous obsession with the Symphonia series, which threatened their family. To protect Kyoko, Kei separated from him and he left the household. Kintaro later reemerged as the antagonist Don Bomby, orchestrating events including the sabotage of Sonata's outfit during a Prism Queen Cup tournament. Kei displays a marked difference in her treatment of family, maintaining coldness toward Kyoko while showing understanding toward Jun, who possesses an intuitive ability to recognize emerging talent in Prism Stars.

Kei adopted Jun after his biological parents died in a nearby car accident, raising him as her son. Kyoko initially believed Jun was her biological half-brother, a misconception corrected when she discovered adoption documents and a photo of Jun's biological parents. Following the Sky High Symphonia tournament and Kintaro's defeat, Kei permitted him to work on her estate's farmland as rehabilitation.

Later, Kei facilitated family reconciliation by having Jun deliver Kintaro's journal to Kyoko. This journal contained the photo confirming Jun's adoption, a revelation that ultimately influenced Kyoko's acceptance of Jun's marriage proposal.