In The Adventures of Tom Sawyer, Jim is the young slave of Tom’s Aunt Polly, the woman who serves as his guardian.

Jim’s role in this novel is brief, especially compared to Twain’s subsequent novel about Huckleberry Finn. In his appearances in this novel, Jim is depicted as a gullible young man on whom Tom is always trying to play tricks. Tom finds enjoyment in torturing Jim, which shows that he is more than just mischievous: Tom is also a bit callous.

Now, the second part of your question has no definitive answer. I suppose that Twain includes Jim in the narrative because slavery was a reality in Missouri at the time in which the novel was written. Twain’s writing style is characterized by its realism and regionalism. He incorporates dialect and cultural realities in order to paint an accurate picture of St. Petersburg. Twain wouldn’t purposefully omit the reality of slavery because that would be falsifying the truth. It also allows Twain to address issues of race in small town America.