It could be due to a variety of reasons. Maybe they start eating more or become less active. Sometimes it's part of the story's development to show a character's change.
It's hard to say for sure as character MBTI types can be subjective. But some might be ESTJ or INFP depending on their behaviors and personality traits.
Typically, there is a main character who is the 'gamer' in the story. This character often has a distinct personality, usually being quite intelligent and adaptable as they need to navigate the game - like world. There may also be side characters like friends or allies who help the main character on their quests.
The main character is the gamer himself. He's an ordinary guy at first who then discovers his new 'gamer' abilities. There are also other characters like his friends who get involved in his adventures. Some of them might have their own special abilities or roles in this game - like world.