The daughter of Judge Thatcher, a prominent person in such a small town. When she moves into town, "a certain Amy Lawrence vanished out of his(Tom Sawyer) heart." Becky Thatcher is found to be very attractive in Tom's view because he gave up on his "engagement" with Amy Lawrence, although Amy had just accepted him.
Tom and Becky have an unstable relationship, towards the beginning of the book, they seem to pretty much hate each other, and towards the end of the book, they actually begin to show some affection for each other.
When their relationship is just starting, Tom accidentally discloses that he was engaged to Amy Lawrence before. This infuriates Becky, and when Tom gives her a brass knob, his most prized possession, to console her, she throws it on the ground. This angers Tom and he storms out. After feeling bad, Tom goes and waits for Becky to come out of school. Then, Becky insults Tom in front of many people. This is one of the reasons why Tom wants to become a pirate.
When Tom comes back from being a pirate, he especially talks to and shows off to Amy, to make Becky jealous. Then Becky, who realizes the whole thing, pretends to be enjoying a picture book with Alfred Temple, one of Tom's enemies. Eventually, Tom and Becky both feel very bad and leave their companions. Alfred guesses that Tom had something to do with Becky leaving him, so he spills ink on Tom's spelling book so that he would be whipped. Becky, initially decides to tell Tom about it, but then decided that a punishment would serve him right. Tom later comes back to apologize to Becky, who denies his apology. Becky then goes into the classroom and finds the teacher's medical book there, because he actually wanted to become a doctor. Becky then starts looking through and finds a shadow over the book. She quickly shuts the book and rips a page in the process, and turns back to find Tom. Becky thinks that Tom will tell on him, so she scolds him and tells him that he was a bad person and was going to get in trouble with the teacher.
When class starts, and Tom's book is discovered, he is whipped after a denial which he is accustomed to doing, and he thinks he might have accidentally spilled some ink. When the teacher asks who tore the medical book, no one responds, so the teacher asks people individually. When it is Becky's turn, Tom "admits" to have doing it to save Becky from a whipping. Becky considers this act to be noble and tells Tom about Alfred.
Later at the picnic, Tom and Becky decide to explore the cave. The next morning however, at church, they are found to be missing. Tom and Becky rest near some water, and slowly come to a conclusion that they are lost. They share a piece of cake that Tom had brought along, and then try to retrace Tom's kite string. On the way, Tom spots the Spaniard, who was actually Injun Joe, peeking his head out of somewhere. Tom, not knowing who it was, screams out thinking that they are rescued, and luckily, Injun Joe is scared away. Finally, Tom and Becky escape from the cave and Becky tells her dad everything that Tom did and how great he is.