How to counter Kai'Sa
Let us take a look on how to beat Kai'Sa. We will look at champions who are strong against Kai'Sa and who are weak against Kai'Sa. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Adc champion who counters Kai'Sa. You can already win in champion select with the right counters for Kai'Sa. If you want to know how to counter Kai'Sa then you need to take into account the best picks vs Kai'Sa and the worst picks vs Kai'Sa. This Kai'Sa counter data is based on matches played by Pros on patch 14.21 and previous ones. It is important to see whether Pros counter picked Kai'Sa or whether they played as Kai'Sa.
Champions used by Pros to counter pick Kai'Sa
Looking at the matches from Pros (see table Pros counter picked Kai'Sa) we can see that they picked 
Jhin most often to counter pick Kai'Sa. The matchup Jhin vs Kai'Sa occured 322 times. Kai'Sa has 170 wins and 152 losses in this matchup. Learn how to counter pick Jhin to get another perspective on the data.
Lane outcome for Kai'Sa
When looking at Kai'Sa counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Kai'Sa. The lane outcome of Kai'Sa consists of three outcomes: Win, Loss or Draw. Let's take a look of Kai'Sa lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Kai'Sa.
Conclusion
Analyze the available data and learn how to beat Kai'Sa. To master Kai'Sa take also a look on Kai'Sa builds and runes for Kai'Sa. Good luck, Summoner!