In the light novel, Kawasaki's house is mentioned to be somewhere close to Hikigaya's house. However, because the middle school district is separated by the highway she went to a different middle school.
According to her brother she was a super serious student back in middle school. She was relatively nice back then and often made dinner. She didn’t change much even when she was in her first year of high school, but she started to change in her second year of high school.
In light novel her debut was on the rooftop of the school. Hachiman wanted solitude to write his future career report, which is blown away by the wind. Saki who is there unknown to Hachiman, catches his report and climbs down to leave. While she was climbing down, her underwear was exposed by the wind. Once down, she glanced at the report and gave it back to Hachiman, calling him an idiot.
One day as Hachiman leaves class, he notices Saki looking at a flyer for summer school classes.
Later, the Service Club runs into Komachi and Taishi Kawasaki. Taishi explains his concern for Saki, since she comes home late every night. Yukino accepts his request for the Service Club and will determine what Saki is up to.
At school, the Service Club tried and failed using various methods to approach Saki. Later on, they learn that Saki works in a cafe somewhere in Chiba City, and track down the two most likely cafes.
The Service Club discover that Saki works night shifts at the second cafe, on the top floor of the Royal Okura Hotel named Angel ladder. Yukino finds that Saki lied about her age to the management so that she can work nights. They fail to persuade her to quit, with Saki arguing that they should stay out of her business. As the Service Club leaves, Hachiman convinces Saki to meet him after her shift.In the morning, Saki meets Hachiman along with the Service Club. Komachi and Taishi, in a nearby restaurant. There he predicts the exact reason for her night shifts and reveals it to everyone. Saki is working so much to fund her summer school tuition, so as not to burden her family. Hachiman also suggests that Saki apply for a scholarship, so she wont have to worry her brother or work nights shifts any more.
In the light novel Volume 5 Chapter 2, it is mentioned that she followed Hachiman's advice and managed to get a scholarship and her relationship with her siblings seems to turned for the better. She still does her part-time jobs only in her leisure hours and spends more time with her siblings.
Later in the series Episode 10 (adaptation of Volume 6) Ebina's eyes fell on the skills of Saki (In light novel by Hachiman's influence) and thus her social relationship improved followed by her participation in various events especially making costumes for class play, athletic festival, etc.
Saki was recruited by Ebina again, this time to make costumes for the Athletic Festival. While she seemed frustrated because of the elaborate costume design, Hachiman thinks she's enjoying herself, being included in plans and working with others.
While Hachiman is at an elementary school with Isshiki recruiting kids for the Christmas Collaboration Event, he happens to find a girl (Keika) that seems a little lost, who is waiting for her sister. Saki shows up to get her, and Hachiman briefly thinks that the girl could be Saki's child. Saki introduces Keika to him, who starts calling him Ha-chan.
Later on Hachiman notices Saki at the Christmas Event watching Keika.
It was later mentioned in Volume 10, when receiving counseling for choosing their career paths, Saki runs into Hachiman where they exchange information about reach other's career choice and also about their siblings. She reveals that she is planning on going to a public school for liberal arts and chose liberal arts courses for her third year.
Differences and Light Novel parts excluded from the Anime
Saki's role in the anime is greatly reduced in comparison to the Novel. There are many misunderstandings and awkward "romantic" moments between the two that are excluded completely.
From Volume 2, her initial introduction on the rooftop is not included in the anime. In Episode 4, Saki is introduced in the classroom and not on the rooftop.
From Volume 5, Saki's attending cram school and meeting Hachiman there. Later they visit a nearby restaurant to see her brother Tashi and Komachi. Hachiman gives advice to Taishi about choosing and getting into a good High school.
Volume 6, Hachiman persuaded Yui and Hina to appoint Saki as the costume designer for the play. But in the anime it was shown to be Hina's own involvement without any influence.
Saki pointed Hachiman towards the rooftop when he was searching for Minami. After Hachiman thanked her by saying, "Thanks! Love ya Kawasaki!", she immediately shrieked loudly in embarrassment and anger after he dashed off to the rooftop.
In Volume 9, She was shown to pick up her little sister from nursery school where she runs into Hachiman. They have a conversation with each other about Saki's improved relationship with her siblings.
In Volume 10 - Chapter 6, when receiving counseling for choosing their career paths, Saki runs into Hachiman where they exchange information about reach other's career choice and also about their siblings.