JM has no interests, no hobbies, no personality, basically there's nothing to him. Other than dancing, he's a pretty empty dude. Borderline robotic. He's either been coached to say these same answers all these years, or as you said he's got zero personality.
When they first debuted, JM was a much more outspoken person. Some of his outspokenness got him in trouble, like the infamous anti-Japanese t-shirt. He was the group prankster, the one to cheer on other members, and so on. Now he doesn't do anything with anybody, and if he is doing anything with another member it's climbing all over them or repeatedly touching them.