In The DC World With Marvel Chat Group在美漫当心灵导师的日子

The MC is explained to have multiple personality disorder.
The MC is acutely aware of this and had explained this at some point between chapters 600-800 if I recall correctly.
The MC says that he has scholar type autism and realized that he doesn't remember things the same way normal people remember things.
He describes his memories as akin to building blocks and from a young age he had used these memory blocks to construct a tower in his mind.
What he didn't realize is that when he made this tower, it's foundation wasn't exactly very sturdy.
So one day it came crashing down, and unfortunately his personality was high in the tower when it fell.
This had caused his personality to shatter to pieces.
So from that day forth he had been trying to piece himself back together.
He used different memories and traits of himself to create entirely new personalities with them. There a two main personality that are most predominant.
The first is "Arrogance" which is the one who we first see in DC, and "Greed" who takes over in Marvel.
There is also the "Super Ego" who oversees everything in the MC's Tower of Thought.
During his first meeting with Bruce, something went wrong with the MC's current personality.
In the DC world, there are certain rules that are almost absolute.
The one that concerns us the most is that whoever Batman takes interest in has a chance of becoming The Joker.
The MC, who was "Arrogance" at the time had made a fatal error.
He caught Bruce's attention when he showed a flippant response to the Wayne family name in his attempt to get Bruce to go away.
This had fulfilled the criteria for the birth of The Joker.
So when The Joker manifested in the MC, he began to mess with Bruce, starting by telling him that the meaning of life is revenge.
Later, the "Super ego" had realized something had gone wrong.
Once the "Super ego" realized what had happened it imminently split the personality that The Joker had manifested from, "Arrogance" in two, locking the other half away.
However, the damage had already been done.
The MC was now entangled with Batman.
Now the first "Arrogance" has to clean up the mess left by the other half which is now referred to as "JokerSchiller."

As for the MC messing with characters in Marvel, the only thing that really needs to be said is that the personality that takes over in Marvel is "Greed". So with a personality centered entirely around one of the most maddening and crazy traits in humanity, it was only an eventuality that "Greed" would start getting himself entangled with characters of Marvel to seize benefits for himself.