Kalista isn't OP
I'll be honest, I'm not very good with her. Nor am I a pro at the game...
But
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She literally only has 2 skills when her support isn't around.
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Her ult doesn't do damage.
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She has no AS steroid.
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Her mobility is only as strong as the number of targets around (Yasuo).
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She's outclassed by most other ADC's in terms of damage.
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She has basically no CC other than a slow that also takes away a big portion of your possible damage in the next 9 seconds...
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and her ult isn't even controlled by her.
I personally think she's balanced and she'll end up just being an average ADC.
P.S... Ezreal seems to be a pretty hard counter to her (We'll see as time goes on)
Feel free to downvote, but she's not going to be a freelo train like everyone keeps saying.