Nagashima was born on November 23, 1991 in Hyogo Prefecture, Japan. Her father is Akihiro Nagashima, a former Japanese national football team soccer player and sports caster. Her mother gave her the name "Yūmi" because she was a big fan of Yumi Matsutoya (called Yūmin among her fans). She has a younger brother. On January 17, 1995, the Great Hanshin earthquake that occurred when she was three years old, completely destroyed her paternal grandparents' house. Announcer Chisa Takeda was her junior in elementary school.