There really is no name for that person. It's just a character that makes life miserable for the main character. She/he doesn't really have a name at all.
the complication
That is called the character's needs or goals. That is also the part that you use to drive the story forward.
The main character, or protagonist, is the one the story's about. The character who is in most of the scenes -- or talked about the most -- will be the main character.
Anything or anyone can be the main character of a story. There are several stories where a house is the main character.
The main character is the one the story is about. Minor characters are characters who support or antagonize the main character.
A protagonist is the "hero" in a story or play. He or she is the main character, the one that the story is mainly about. He is opposed by the antagonist or "villain."
The main character is known as the protagonist.
The main character in a story is the protagonist.
Assuming that you mean the main character of any story, it would be the Protagonist.
The main female character of a story is usually called the main character. In Epics the main female character is called a heroine.
The main character was Ralphie Parker, who was played by Peter Billingsley.
The main character will be the person that the story is about, either the hero or the villain. Your teacher wants you to think about the story and decide which character is the most important to the story.