Kai'Sa is an Assassin Marksman who has good scaling with high dmg numbers and good mobility.
You spike hard at 2 Items and you spike with your evoolves (Q Evolve at Kraken + 2 Long Sword, E evolve at Kraken, boots, PD).

Your early game is pretty potent when paired with engage like nautilus, leona etc.
Your W does more dmg based on how much plasma stacks are on the enemy so if ur naut goes in hooks and roots him you can hit W for free with big dmg. At lvl 6 your all in gets really good and you have insane follow up with ur ult.
In hard lanes u can focus on farming to get your items.

Mid game is where you shine with finding picks and catches. Always look out for opportunities to follow on your teams engage. But keep farming so you dont fall off.

Your late game isnt as strong as other crit ADC's in the meta cuz ur low range but with the right teamcomp you can dive enemy backline and oneshot their carries.
Your dmg to danks is really good after LDR and ur passive you are able to play front to back until you find an angle to get into backline.

Kai'Sa is a playmaking hypermobile Assassin with good scaling.
You have all the tools to carry a game singlehandedly. Kai'Sa isnt really hard to play but you need to know your limits to perform well.
Once u get the hang of her interactions she is really fun to play, especially with a Duo partner.