As with any author's decision to use 1st person, her intention most likely -- in this story -- was to get her readers to go along for the ride into madness and cultivate a certain amount of sympathy for the narrator and her plight. The constant use of "I" puts readers in the narrator’s head and allows them to empathize with her.
A reliable source is a document!
A foil is a character whose traits are distinct or different from another character. Rainsford's character is highlighted by its difference from Zaroff's character. Zaroof is ruthless and cruel while Rainsford is compassionate and values life. You could say that their characters are the complete opposite.
Grandpa told me about his life growing up; when he was just 16, there was a job opening at the nearby car wash. His Mom and Dad wanted him to go and apply for the job, so he did. Grandpa Frank walked down the street and sat down for his interview. Then, the boss’s daughter walked out and sat in front of him. “What’s your name?” She asked. “Frank, and yourself?” He replied. “Grace, nice to meet you.” She smiled. Grandpa Frank was already noticing how beautiful the girl was. He wanted the job just to talk to her. The interview went on and Grandpa Frank went home until he was called a few days later being told he got the job! He was so excited & quickly went to work the next morning. Everyday he worked; Grace would talk to him at his lunch break. It was their own little dates. A few years later the car wash was shutting down due to going bankrupt. Grandpa Frank didn’t want to loose Grace; so on a cold, starry Tuesday night, he got on one knee & asked Grace to marry him. Grace quickly agreed then got married the next week. That’s the story how Grandpa Frank met Grandma Grace.
