How are Harriet Wheelwright, Tabby, and Dan important?
Harriet Wheelwright, the narrator's grandmother, is the matriarch of the Wheelwright family, and for that matter, of the town. Her husband died young and left her not only a considerable amount of money, but a fairly considerable reputation. She herself, according to the narrator, is a descendant of John Adams, and her family originally came to America on the Mayflower. As a result, she is prim and proper (though very loving) and she lives in a very large house. What is most important about her is her distinct connection to the institution of the name "Wheelwright." It is as if she is the last one of the Wheelwrights to truly embody the meaning of the name in the town. Consider the following (from pgs 6&9):
Let me say at the outset that I was a Wheelwright--that was the family name that counted in our town: the Wheelwrights. And Wheelwrights were not inclined toward sympathy with Meanys...And I never suffered in those years that I had her name [meaning his mother's]; I was little Johnny Wheelwright, father unknown, and--at that time--that was okay with me.
Tabitha Wheelwright, the narrator's mother, is only alive for the very beginning of the book. Unlike her own mother, she is much more unconventional, and does not seem to uphold the same standards of the reputation of her last name. She is both prim and proper, but in a different, more modern way. She has a child out of wedlock, for example, and keeps the father's identity a secret from everyone, including her son. Furthermore, she is young and very attractive mother. She does nothing to downplay her physical beauty, a fact which causes the men's heads in town to turn, the women to be suspicious and jealous, and Johnny's friends (including Owen), to lust after her. Owen, of course, has more than lust for her beauty. He confesses throughout the book that he is completely in love with her, and likely this is true, which is why, when it is his foul ball that kills her, he feels an indelible need to sacrifice something of himself for his friend Johnny. Tabitha provides a personal connection between Johnny and Owen for life.
Finally, Dan Needham, Tabitha's new love interest, is in important constant throughout Johnny's life. Though they are leery of him at first, Dan quickly grows on both the boys (with the introduction of the stuffed armadillo) and after Tabitha's death, Dan legally adopts Johnny. Though a man of few words, he seems to have a natural sense of understanding about him, to those around him, especially his students (including Johnny and Owen). Dan's constancy is what is important about him, throughout the story.