The main character is probably the man who has a connection to the rats. There might also be other characters like people in his community who are affected by his relationship with the rats. For example, if he's a rat catcher, there could be clients who hire him to get rid of the rats in their houses.
Well, if the 'rat man story' is about a man who is cursed to become part - rat, part - man, then he is the main character. But there could also be other characters like the person who cursed him, or his friends and family who react to his transformation. Maybe there are also other animals or people in the environment where the story takes place, like the townsfolk who are terrified of the rats or the rats themselves which could be seen as characters in their own right if they have some sort of intelligence or agency in the story.
Since we don't know the exact 'rat man story', it's hard to say for sure. But likely the main character is the 'rat man' himself.