She is particularly close with Hamagishi Hiyori. On a segment on HINABINGO!, she chose Hiyori as her ideal little sister and Hiyori chose her as her ideal big sister
She often texted Hiyori while Hiyori was on hiatus. Hiyori claims that if Mirei had not reached out to her so often, she may have decided to graduate from Hinatazaka46
Mirei once borrowed a manga from Hiyori and returned it with a stick of Mintia gum. She had been using the stick of gum as a bookmark
Shares a family name with Sasaki Kumi. Audrey's Wakabayashi, one of the hosts of Hiragana Oshi and Hinatazaka de Aimashou called them by similar names (Sasami and Sasaku)
Can play the violin
Revealed in 2021 she's taken to drinking the herbal wine Yomeishu (養命酒) every day, to maintain her health and performance. She sometimes drinks it at work, drawing comparisons to Showa-era actors from Audrey. The MV for Suppai Jiko Kenno alludes to this by featuring her working at a liquor store