Does Lina Lamont or Kathy Sheldon use a characters voice in singing in rain?

In "Singin' in the Rain," Kathy Sheldon is the character who uses her singing voice to dub for Lina Lamont. While Lina has a distinctive speaking voice that is famously shrill, she cannot sing and relies on Kathy's vocal talents to perform in the musical film. This contrast between their voices serves as a key plot point, highlighting the challenges of transitioning from silent films to "talkies."