From TARDIS Index File, the free Doctor Who reference.

Lexa was the leader of the Deons, the religious Tigellans who worshipped the Dodecahedron. When Meglos, disguised as the Doctor, stole the crystal, she tried to have the real Doctor executed. She eventually realized that the Doctor was innocent, and was killed by a Gaztak while saving Romana's life. (DW: Meglos)