The top champions globally to counter Kassadin include Zeri, Kalista, and Sivir. These are all good at countering Kassadin with win rates over 60%. Even if these Kassadin counter picks are not played opposite your position, it's not a good idea to choose him into any of these matchups. They will most often destroy Kassadin.
Yet, if the other team brings either Karthus or Kled, Kassadin mains should strongly think about picking him, as he easily wins against these selections with win rates over 56% against both ofthem.
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