Rhea

Supergirl
Rhea was a member of the Daxamite race of the planet Daxam. She was the queen of the planet and ruled with her husband, Lar Gand, and her son the prince, Mon-El. When Daxam suffered great geological upheaval due to shock-waves from the explosion of nearby Krypton, the royal family were forced to flea. Mon-El parted ways with his parents and eventually came to Earth. Queen Rhea and King Lar Gand as well as many Daxamite survivors came to Earth with the intent of terraforming it, so they can establish a new home for themselves. This method would naturally make the planet unlivable for humans though. Supergirl collaborated with Lena Luthor to create a device that would make Earth's atmosphere intolerable to Daxamites. Doing so however, meant that Mon-El would be forced to leave Earth. There was no other recourse however, and Lena Luthor activated the device. Rhea perished at this time.