Movie She Appears In: Men in Black II

What’s Her Evil Plan? She's the queen of the Kylothian military, spending over two decades looking for the Light of Zartha - a cosmic force left behind by a Zarthan princess. Agent K sends her on a wild goose chase.

What Is She? Leader of the Klyothian from the planet Jorn. She's an attractive vixen in her human form, and her alien form is a plant-like snake. She has plant tentacles, vocal replication, a prehensile tongue, and regeneration.

How She Meets Her Demise: Jay shoots her, but her wormy form survives, only to become worm food for Jeff. But she still isn't gone. After becoming a creepy worm-tentacled hybrid thing, Kay serves the killing blow. 

Grossout Factor: 1 (her human form is pleasant and she's not too creepy in her alien form, either - but the bar is low on that one)