A doctor who is not a doctor, with fangs, a skin she can shed and a poison she can breathe on you.
Viper is a mutant with reptilian features: fangs, the ability to shed her skin, and toxins she can expel from her mouth. Svetlana Khodchenkova plays her in The Wolverine, where the cast table on Wikipedia lists her as Dr. Green, the name she goes by while posing as the Yashida clan’s physician.
Her one decisive act comes early. She poisons Logan in his sleep with a nanobot, and the rest of the film is about a Wolverine who cannot heal.
Minor spoiler
Yukio kills her by breaking her neck with an elevator’s counterweight.