Any idea about what an integral character is in literature?

I believe the first step in analyzing an integral
character is literature is to define it. Merriam-Webster.com defines "integral"
as:

essential
to completeness

In terms of a
literary character, I believe these characters are referred to, generally, as major or
minor characters.

Dr. L. Kip Wheeler defines
characters—

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The main character of a work of a fiction is
typically called the protagonist; the character against whom the
protagonist struggles or contends (if there is one), is the
antagonist. If a single secondary character
aids the protagonist throughout the narrative, that character is the deuteragonist (the
hero's "side-kick").

However,
an integral character could be a main character or a minor character. It is more the
purpose he (or she) serves that is important.

My sense is
that an integral character is an essential character in a story or
play. For example, in Romeo and Juliet, the servants present while
the Capulets prepare for Juliet's wedding are secondary, supporting characters, but not
essential to the development of the plot. Romeo, Juliet and even Friar Lawrence are
integral characters, as it is around their actions that the plot evolves, while they are
also major characters.

In Hamlet,
Horatio is an integral character, as is Fortinbras (who does not even appear until the
end of the play), but neither is what I would consider a major character. Horatio
provides someone through whom Hamlet speaks to the audience as he tries to work out his
indecision over avenging his father's death. Horatio is the one person Hamlet trusts in
the castle. Fortinbras serves as a foil. His actual stage role is very small, but
knowledge of this character is woven through the story; Fortinbras has a great deal in
common with Romeo, also as the son of a dead king, and so we are able to compare the
reactions of Hamlet as opposed to those of Fortinbras of
Norway.

Some integral characters may serve more of a
purpose in the context of when their story or play was written. In Romeo and
Juliet, the Nurse is the source of comic relief. In
Macbeth, the Porter is the source of comic relief. We may not
readily understand their humor, but the Elizabethan audience would have. This might have
made them integral in these tragedies to give the audience a
"breather."

In To Kill a Mockingbird,
the "character" of Tim Johnson is an integral one. Tim Johnson's appearance one summer
afternoon allows the reader to learn that Atticus, though he hates guns, is an
exceptional marksman. The concept of protecting mockingbirds, as reflected in the
novel's title, is also introduced by way of Tim Johnson's presence, a theme that runs
throughout the novel.

The distinction of the integral
character is not that he/she is a major character, but that his/her role is essential to
the development of the plot.