Placing wards over walls near lanes instead of lane bushes will be more effective at spotting Kayn as he can use his E to avoid common warding spots.
Kayn will look for early skirmishes so he can get his form as quickly as possible. Expect him to invade or gank a lot in the early game.
Because of Kayn’s Ultimate R and mobility, make sure you watch yourself as you recall, and make sure you stand in the middle of the lane so it’s easy for you to escape if he springs up nearby. Don’t give him easy kills by window shopping when you recall.
Kayn will try to assassinate targets that are alone during the mid-game. Avoid walking through the river alone if you’re unsure of Kayn’s location.
Kayn can tower dive while sieging thanks to his E. Make sure you have your flanks warded so you can spot him before he jumps onto you or your team.
Shadow Assassin will look to nuke your backline by flanking or going through walls. Try to peel your backline or set up vision so that you know where he is at all times.
Kayn is rather good in the early game and will look to invade/gank as frequently as he can so he can get his transformation.
When Kayn gets his transformation, he will be much stronger. If he goes Red Kayn, he will be tankier, if he goes Blue Kayn, he will deal tons of damage.
At level 6, Kayn can be more aggressive and go for riskier plays as he can use his Ultimate R to protect him from dying. Keep this in mind when fighting him.