Infer: Why would Mark Twain include Sid in the plot? (Hint: Character foil) *Chapter 3

Sid is Tom's younger half-brother. Always trying to tattle on Tom, Sid keeps a close on eye his brother's wrongdoings. A goody-two-shoes, he is a punctual and studious pupil. Tom is the opposite. Character foils bring out the traits of each oppsing character.