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Lexa was the leader of the Deons, the religious Tigellans who worshipped Ti.
Lexa believed the Dodecahedron a gift from Ti. Therefore, she controlled who was able to enter the room in which it was kept. This angered the Savants, a scientific group who wanted to examine it.
When Meglos, disguised as the Fourth Doctor, stole the Dodecahedron, she tried to have the real Doctor executed. However, Zastor did not want to punish him as he believed him innocent. Lexa was angry at the decision, so she had the Deons stage a revolution. They tried to sacrifice The Doctor but he was freed and escaped.
Lexa was convinced of The Doctor’s innocence and said her farewells to him as he left to pursue Meglos. An apparently dead Gaztak rose momentarily to shoot her. She died soon after.. (Meglos)