Two things to keep in mind when you're deciding whether to use formal or informal english are
a. the author and the audience.
b. the occasion and the audience.
c. the reader and the location.
d. the opinion and the occasion.

a biography is a genre that mainly accounts somebody's life and that is written by somebody else. this genre supports sonia nazario's desire to share enrique's life because she can fully inform readers of his entire emotional journey.

in a biography, an author can share the readers almost everything about a person's life, or just one key event or personal aspect the author is interested in telling, like in this specific case in which sonia nazario is centered in enrique's emotional journey.