Well, they relate to each other in how they effect each other and how they are parts of each other. If a character is outside of a plot and not part of it, it still affects the character by the fact of observation. If the character is the narrator, the word choice might indicate whether he or she approves or disapproves of what is going on, etc. The plot relates to the character in that it is the story of characters... and relies on characters for existence. Also on a good author... but that is another question entirely. :)
A protagonist is the main character in a story, typically the one who experiences the most significant character development or drives the central conflict of the plot. They are usually seen as the hero or the character the audience is meant to relate to or root for.
Interpersonal traits are traits relating to how people relate to one another. Some interpersonal traits are temperament, character, and morals.
The answer is asking which of the character's traits or situation is similar to that of your own. The way that I would suggest to answer it is, "I can relate to (character) because I was in a similar situation once..." or, "I can relate to (character) because I was also (trait) at one point.
help people relate better with one another.
A plot-based story is one that is told so that the plot, or what happens, is the most important thing in the story. Adventure stories are plot-based. Character-based stories are those where the characters are the most important parts. Series books are usually character-based.
Not necessarily. The plot is the main storyline. For example if a books character needed to defeat an evil enemy but at one point had to do training with an unknown teacher it would be a kind of 'side story' although it does weave in with the main plot. Or the story might switch from the main character to another characters point of view so that you could see what they were doing. Again this will probably weave in with the main plot but it is still not the main plot.
A secondary character is a character in a story who is not the main focus, but still plays a significant role in the plot or development of the main characters. They often serve to support or contrast the main character and add depth to the story.
You begin with "Dear ___ (put the other character's name)" and just write a letter. If you can't think of what to put in, perhaps writing a letter is not the best way to connect these things! Very few writers use letters as a way to advance the plot.
system
Trigger and move the plot and their various relationships to one another form the basis for conflicts and dynamic process
because they can relate to one another
When you're writing fiction, you start with one of two things - a plot or a character. Either are good. When you start with a character, you imagine different things that might happen to that person; when you start with a plot, you imagine what characters would work best with that plot.