Sorry to all Lloyd fans but I couldn’t resist doing Kai and Morro, I love their parallels too much.
My interpretation of how a "Morro possesses Kai au" would be:
At first it's the usual possession but it's very violent since Kai has a bad temper. Morro agressively possesses him and breaks into Kai’s memories against his will.
But Kai is stubborn and angry and doesn't let Morro hurt Lloyd. So after the fight Morro digs deeper into Kai’s memories to see what he can use against him.
Morro finds himself relating to Kai and tries to manipulate him to betray the ninja and work with him (going as far as to showing Kai his memories)
Kai still refuses to hurt the ninja and shifts the narrative "why go after Lloyd when you can go after Wu?".
Kai basically learnt what Wu did to Morro and him and gets angry and wants to get revenge on him.
Morro is conflicted into what Wu did and his love for his only father figure. But Kai is also conflicted because on one side he relates to Morro but on the other Morro tried to use his body to hurt Lloyd. In the end I think Kai would choose his anger towards Wu and try to convince Morro even if he has to do so by manipulating him because as much as he's empathetic, he knows he can’t save Morro, and can’t get easily over the fact he wanted to harm Lloyd.
It would be interesting that at some point Morro had to fight Kai because he wanted to attack Wu and the ninja of course misinterpret it and still think of Morro as a villain.
To sum up they'd try to manipulate each other and end up exhausted from fighting the possession.
I love exploring the duality of broken villains.