In this book, Rontu changes Karana’s life mainly by making her much less lonely than she had been before she got him.
By the time that Karana gets Rontu, she has been alone for months. The dogs had killed her brother Ramo, leaving her completely alone. She had burned down the village, symbolizing her complete break from the life she used to know. This is, of course, a very hard position for her to be in.
When she gets Rontu, that all changes. Now she once again has someone to care for and to keep her company. She goes all over the place with Rontu. He even protects her to some degree by driving the wild dogs away. Thus Rontu really changes her life psychologically and emotionally because he gives her a companion to care about.