Harriet is the illegitimate daughter of an unknown someone—the identity of her father is revealed later. He had placed her, years back, at Mrs. Goddard's boarding school.
Mrs. Goddard wanted to bring her to Hartfield estate for tea with Henry Woodhouse and his younger daughter, Emma. This was welcome, as Emma had always wanted to meet Harriet because of her beauty. She was excited, and also very nervous, to meet the Woodhouses, particularly Emma, who was such a well-known personage in Highbury. Her concerns were soon put to rest by how genial Emma was, and Harriet was especially happy that Miss Woodhouse had shaken hands with her.
Emma continued to invite Harriet to Hartfield, and thus succeeded in finding a new good friend to help her get over Mrs. Weston's departure. Emma was dismayed to discover that Harriet had developed feelings for Robert Martin, a farmer who lived near Donwell Abbey, as she thought the match and relation generally unsuitable.
Emma attempted to move Harriet's interests towards Mr. Elton, the local vicar. Harriet sat for a portrait by Emma, and Mr. Elton enthusiastically helped with the task, even volunteering to go immediately to London to have the portrait framed. Upon his return, the portrait was hung at Hartfield. The friendship between Emma and Harriet grew, the two regularly walking together and socialising at Hartfield. The two became interested in charades, lyrical riddles with one-word solutions. One day, Mr. Elton sent over a charade, saying it was one of his friend's. While Harriet puzzled for a bit on its meaning, Emma quickly concluded that the solution was "courtship" and took the charade as a sign of Mr. Elton's growing fondness of her. On one of their walks, Emma left Harriet with Mr. Elton at the vicarage to socialise and became convinced at the match after she saw them talking, cementing the pair's belief of Mr. Elton's attraction.
Unfortunately for Harriet, she developed a cold near Christmas and stayed at Mrs. Goddard's school over the holiday. After a Christmas party at Randalls, Emma and Mr. Elton were forced into a carriage together, and Mr. Elton confessed his love for Emma. Emma was shocked by this, inquiring if he had drunkenly mistaken her for Harriet. Mr. Elton was, likewise, shocked at this suggestion, exclaiming that he and Miss Smith are of two different levels. As it turned out, the actions that Emma was so sure was evidence of his attraction to Harriet was actually aimed towards herself. The rest of the carriage ride passed awkwardly. Miss Woodhouse informed Harriet after the fact. While she cried heavily at the news, she "bore the intelligence very well, blaming nobody."
She is described a being quite pretty, short, and plump, with a fine complexion, blue eyes, and blonde hair. She was well-mannered, and impressed Emma with her sweet-natured personality.
She is also unsophisticated and is too easily led by others, especially Emma. Emma takes Harriet under her wing early in the novel, and she becomes the subject of some of Emma's misguided matchmaking attempts. Harriet initially rebuffs a marriage proposal from farmer Robert Martin because of Emma's belief that he is beneath her, despite Harriet's own doubtful origins. She then develops a passion for Mr. Knightley, which is the catalyst for Emma realising her own feelings. Ultimately, Harriet and Mr. Martin are wed, despite Emma's initial meddling. The now wiser Emma approves of the match.