Kim Sertori is Cleo's annoying younger sister who is always snooping around Cleo's room and is also always leaning against Cleo's door with a glass and hearing everything she says. She was portrayed by Cleo Massey.
She was close to discovering the girls' mermaid secret once in the first season. That is when Kim finds Cleo's Diary in which she has written all things to do with mermaids. When Kim finds this out, she tries to expose Cleo, Emma and Rikki as mermaids at the Miss Sea Queen pageant in the Sea World Marine Park. Kim also believes that Miriam is their mermaid leader, and another follower is Miriam's friend Tiffany. Cleo, who is on the stage dressed as a jellyfish, protects herself with her power while the other girls' costumes are ruined. Then, Cleo tells Kim the diary was a homework assignment about "The Little Mermaid" and the tail was a costume.
During season three, Kim is more spoiled and bratty than in the previous two seasons because she thinks she can do whatever she wants and can be the girl who has more than one boyfriend. She even has a pink, sparkly, jeweled cellphone. Kim hates Sam, even when she gets married to Don by stealing the rings and turning on the sprinklers at their wedding and soaking Cleo, Rikki and Bella. Which later Cleo fixes her dad's wedding by making it a Mako Island only smaller. This episode also shows Lewis departing to America to a school of science. That's when later Kim changes her mind about Sam and the marriage.
At the beginning of the series, Kim has wispy-looking bangs and generally chooses to plait her hair. She often ties the braids with wide bands that match her outfits. She seemed to be girly, often wearing pink or purple garments. She seems to share her sister's taste in clothing, as she often "borrows" Cleo's outfits. She has a round, freckled face with a button nose and eyebrows a shade darker than her honey blonde hair. Throughout the series, Kim physically (though not mentally) matures quite a bit. She grows taller, and her body starts to show signs of womanhood. Her hair seems to darken a few shades, and she grows out her hair, choosing to wear it long and loose, sans fringe. Her taste in clothing also evolves, as she begins to prefer shorter skirts and more stylish tank tops over her previous preferences.
Kim's role is the series is a minor villain one, intended to serve as a source of constant conflict and friction, in both the girls' home life and their mermaid life. She is incredibly nosy and likes to snoop on her sister. Her favorite pastime seems to be vexing her elder sister and she always is coming up with new schemes. In addition, Kim is also bossy, annoying, manipulative, and extremely selfish, only looking out for herself and not caring who she has to hurt or manipulate to get what she wants. She is also shown to delight in seeing her sister humiliated or wronged. This is not limited to just Cleo however, as she has shown the same delight in seeing Sophie fire someone unfairly, suggesting she admires other people who are corrupt like she often is. Occasionally, Kim displays a softer more sensitive side. She is shown to be greatly saddened by hers and Cleo's mother leaving expressing feelings of loneliness. In spite of her usual cruelty towards her sister, she does care for her, and was greatly distressed when Cleo ran away from home.
- Main article: Kim Sertori (JAW)/Gallery