Kupper was born at Glatz (now Kłodzko) in Lower Silesia.
She studied in Breslau and was a music teacher there before making her operatic debut in 1935.
She sang Eva in Die Meistersinger at the Bayreuth festival, in 1944, and returned as Elsa in Lohengrin in 1960.
Kupper was especially admired as Countess Almaviva, in addition to Wagner and Strauss roles, she also gained considerable acclaim as Aida and Desdemona (in Otello).
A sensitive and warm-voiced singer, she retired in 1961, and taught at the Music Conservatory in Munich.