How does this line from the story the reader better understand bertie’s character? do not lose sight of the fact that all this was taking place on an empty stomach, shortly after the rising of the lark.

It reinforces the idea that Bertie is not happy about Aunt Agatha's visit.

Explanation: In Chapter 2 of "The Man with Two Left Feet" by P. G. Wodehouse, Bertie is visibly annoyed with Aunt Agatha's visit. He already hinted he was not in the mood when he stated "I was feeling like a badly wrapped brown-paper parcel. I'm never at my best in the early morning. I said so."

On top of him not being a morning person, and dealing with an unwanted visitor, he was also hungry. He is expressing to both Aunt Bertie (and the reader) that is not in the mood.

figurative language is a resource that fiction writers use to make better descriptions of what they want to write about or express. they can improve their stories, as they can elaborate more vivid and colorful sentences. this way, the reader will be able to have more detailed images in his/her mind.

places, objects, people and even feelings or emotions can be described with a lot of detail to metaphors, similes and personifications. making different kinds of comparisons to make a description more specific, as well as assigning human qualities to non-human objects (and vice-versa) can be very useful for science fiction writers.  

on the other hand, alliterations allow writers to elaborate more entertaining and creative sentences. it consists in using a sequence of words that start with the same consonant letter. they can be used to emphasize something important the writer wants to express, or just as a fun literary device.