How to counter Katarina
Let us take a look on how to beat Katarina. We will look at champions who are strong against Katarina and who are weak against Katarina. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Mid champion who counters Katarina. You can already win in champion select with the right counters for Katarina. If you want to know how to counter Katarina then you need to take into account the best picks vs Katarina and the worst picks vs Katarina. This Katarina counter data is based on matches played by Pros on patch 14.23 and previous ones. It is important to see whether Pros counter picked Katarina or whether they played as Katarina.
Champions used by Pros to counter pick Katarina
Looking at the matches from Pros (see table Pros counter picked Katarina) we can see that they picked 
Sylas most often to counter pick Katarina. The matchup Sylas vs Katarina occured 28 times. Katarina has 16 wins and 12 losses in this matchup. Learn how to counter pick Sylas to get another perspective on the data.
Lane outcome for Katarina
When looking at Katarina 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 Katarina. The lane outcome of Katarina consists of three outcomes: Win, Loss or Draw. Let's take a look of Katarina 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 Katarina.
Conclusion
Analyze the available data and learn how to beat Katarina. To master Katarina take also a look on Katarina builds and runes for Katarina. Good luck, Summoner!