Viper is the secondary antagonist of the 2013 X-Men spin-off film The Wolverine. She is a mutant, whose powers include being able to spit an acidic venom at people, growing out her fingernails, and being able to harden her skin into armor-like scales, which she can then shed. She was played by Russian actress Svetlana Khodchenkova.
Why She Sucks
The original comic version of Viper wasn't even an X-Men character to begin with, but rather one from Captain America. As a result, she has almost nothing in common with the comic version, who wasn't a mutant.
Her powers are either unimaginative, in the case of being able to turn her fingernails into claws (something very similar to the powers of earlier film characters like Sabretooth, Lady Deathstrike, and, oh, Wolverine himself!), or just really weird, in the case of her skin-shedding power. Her ability to spit poisonous acid seems to have been ripped off from, of all things, an episode of Star Trek: The Next Generation where an alien virus-transformed Worf into an acid-spitting monster.
She has zero personality beyond being just a generic evil chick who enjoys poisoning people. She's basically like the Batman & Robin incarnation of Poison Ivy - to the point where she even kills two bystanders by fatally poisoning them with a kiss - only without all the horrible plant puns (which are replaced by a thankfully much smaller number of puns relating to lizards and poison).
As with several other antagonistic mutants from the X-Men film series, we're never given a motivation for any of her actions other than the fact that she hates all non-mutants, and is working against the heroes just for money.
Svetlana Khodchenkova is seriously miscast as Viper and gives one of the worst performances in the entire series.
She's indicated to be a medical genius with the power to stop Wolverine's regenerative powers from working, but never displays this kind of knowledge again after her introductory scene, and doesn't think to use it to somehow neutralize Wolverine when she has him captive at the movie's climax.
Her death scene is pretty anticlimactic; the two "fake-out" death scenes she gets earlier in the climax were actually better by comparison.
Her outfit's at least quite faithful to the one her comic counterpart wears, even if it's a little on the cheap side.
The character would later be done much more faithfully on Agents of S.H.I.E.L.D., although it was the sentient Life Model Decoy Aida when she was using the Framework.