She is mentioned in seasons one and two, but is not introduced on screen until the season three episode "Back to School Blues", where she spots D.J. and Kimmy Gibbler on their first day of junior high. She is been in school with both girls for six years, but they do not recognize her, due to growing up "big time", according to D.J. Kathy then attributes her looking taller and having large breasts to a "pretty intense summer". The next day, after D.J. tries to look older by putting on makeup and wearing skimpy clothes to show that she is indeed a "young woman", Danny tells her that she cannot go from 12 to 25 overnight. D.J. tells him that Kathy Santoni did.
Often, she is seen as either a friend of D.J.'s or an enemy, depending on the situation. Despite being the reason D.J.'s very first boyfriend broke up with her, Kathy always tries to help her out. For example, for D.J.'s 13th birthday, she and Kimmy devised a plan to get her and Kevin Gwynn together (which backfires but earns D.J. her first kiss). D.J. can get jealous or angry at Kathy sometimes, or can make her angry at other people, such as the time when her boyfriend, Steve, gives Kathy a ride while she is wearing "fifty pounds of makeup". D.J. even asks Steve if he thought Kathy had bouncier hair than hers.
Kathy is said to be pretty by D.J.'s first boyfriend, and apparently other boys think that too. At the Backwards Dance, she has three dates, which proves the point that a lot of boys like her.
She does not seem to be the brightest. As D.J. put it in the season six episode "Radio Days": "Kathy Santoni is such a bimbo! She even signed up for shop class 'cause she thought it was taught at the mall!"
During Career Day (when Danny and Rebecca are talking about their career instead of Connie Chung, who was supposedly thought to have come), she asks how tall Connie Chung was, even though she isn't there.
In "The Apartment" from season seven, D.J. and Kimmy reveal to Danny that Kathy is pregnant (Kimmy also adds that Kathy's actually getting extra credit for in their Home Economics class). Later in that episode, Danny apologizes to D.J. for freaking out and treating D.J. like a little kid, because, as he put it, "All I kept thinking about was 'Kathy Santoni, a girl your age, having a baby'." D.J. assures her dad that she is not like Kathy and that there are lots of things she wants to do before she eventually has a family of her own, which makes her dad feel better.
- Kathy is the only character on the show that is mentioned more than she is actually seen
- In one of ABC's "One to One" PSAs, it is revealed that she made the JH cheerleading squad on the exact same day D.J. tried out and did not make it (toward the end of season 2)
- In the Fuller House season one finale, Kimmy suggests calling Kathy after she, D.J., and Stephanie leave a message on Michelle's voicemail
- The producers were looking to recast the role as part of a high school reunion episode in season two, however, the original actress agreed to return to reprise her role