Lewis McCartney is a teenage boy who has been Cleo's friend since they were five, and following the girls' transformation, also becomes friends with Rikki and Emma. He is academically gifted, thinks of himself as a scientist and has a passion for fishing. He has four brothers, one is named Lenny but three of them aren't mentioned.
Lewis was the first to discover the girls' secret early in the series at a pool party when Cleo is thrown into the pool by Zane and his friends. Lewis was the only one there when Cleo transformed, at first he was scared, but then he helped her. Emma and Rikki also revealed themselves to him as mermaids and he becomes determined to find the cause. Lewis helps the girls in their situations such as the full moons. In series three, Lewis becomes good friends with Bella Hartley, who is another mermaid from Ireland and a new best friend of Cleo and Rikki.
He departed for America to study science after he received a scholarship from a university, but came back for graduation.
Lewis' personality is unique compared to most of the other male characters in the series, He is talented, smart, and always willing to help out others when they are in need of assistance as well as being reliable and can always keep a secret no matter what. This led Emma, Rikki, and Cleo to have Lewis' trust of their mermaid secret. Most of his time is spent helping the girls in tricky situations when ever it comes to their mermaid secrets and protecting them as well as trying to discover the cause of their transformation.
Lewis likes to think of himself as a gifted scientist and enjoys building many types of inventions such as weather analyses and recycling units which he has tried to win 'young inventor of the year of award' once. He also has a taste for fishing that he often does in his past time as well as inventing his own fishing lure. He also takes part in fishing competitions. Lewis can be pompous and somewhat controllable at times which means he can get on people's nerves, such as Rikki who tends to mock him about his scientific explanations, saying that they're pointless and distracts her from her social life. Another example is in the episode "Riding for a Fall" when he built a food recycling unit and was demonstrating it in the Café, Wilfred became angry with him because it was scaring his customers away. Because he is well organized, he and Emma get along well with each other because of their relatively similar personalities.
On occasions, Lewis can be considered to be somewhat of a loner, since he does not interact with many guys. He and Zane have spent time with each other to help out the girls with their secrets, but are not exactly close friends. In one episode, Nate told Lewis that because he spends most of his time around girls he begins to act like one. Lewis is not a stereotypical guy and does not get involved in sporting hobbies such as surfing, motorbike riding and diving like Zane, Nate and Will do. Still, Lewis does not tend to mind this and is his own self with Emma, Cleo and Rikki as his friends. Because of his caring, and friendly personality he is more acceptable and liked by many people (except Nate). In the episode "Young Love", when Rikki and Emma ask if they will be paid for getting him equipment for his brother's plant nursery, Lewis replied "Only with my caring friendship".
Lewis and Cleo have been friends since they were young, and have each others trust and faith. Lewis is the first person that Cleo would turn to for advice, and he has helped her out on many occasions. Throughout the first season they were shy to let out their interest for each other until they revealed their likings for each other and then they both become a couple, but shortly afterward they break-up in the second season, but stay friends and remain a little embarrassed.
He began dating Charlotte after his break-up with Cleo which made things very difficult to help the girls and spend time with Charlotte. When Lewis and the girls discover that Gracie was Charlotte's grandmother that leads her to Mako Island and learns the girls' secret, she steals Cleo's locket and says that Lewis would have nothing to do with her anymore. Lewis breaks up with Charlotte after discovering this and goes to look for Cleo who ran away because of it. After finding her, the two of them renew their relationship with a kiss.
In the 3rd season, Lewis and Cleo continue to be together. They also try to figure out how the water attacks the mermaids during the full moon. Lewis is given the chance of a lifetime with an opportunity to study overseas. But he's interrupted from telling Cleo by Sam and Don surprising everyone and announcing an early wedding ceremony. When Kim learns about Lewis' impending departure, she sees an opportunity to sabotage the event and breaks the news to Cleo. Distraught but determined, Cleo focuses her attention on organizing the entire ceremony for Don and Sam on her own, outside the café. With Lewis as Best Man and Cleo as Maid of Honor, the ceremony is icy. Another stunt from Kim gives Lewis the opportunity to declare his love for Cleo and let her know his decision — he's not going. Not to be foiled by her sister, Kim plots yet another way to derail everything and this time succeeds, nearly exposing the trio as mermaids. But with Rikki, Bella, Will and Lewis' help, Cleo recovers the ceremony and transports it to Mako, to finally give Don and Sam the wedding they deserve. In the process, she realizes she must let Lewis go to follow his dreams in America — and in a bittersweet goodbye, Lewis departs.
Though they have parted ways, Cleo and Lewis remain in contact and still in a relationship with each other. Lewis returns to visit in the episode "Graduation", where he and Cleo kiss and dance to Bella's singing.
Lewis doesn't hear the message because Charlotte turned off his phone off and stole it. After hearing the message and hearing that Cleo ran away, he runs to his boat. Charlotte tells Lewis that he had to choose to either go and find Cleo or stay with her. Lewis chooses to break up with Charlotte and goes to Max. Before going to Max, he breaks up with Charlotte and not realizing what an enemy he has created against himself, and the other three girls. Max tells him that he should look in a reef, which was a shark breeding ground, because Gracie used to go there when she was upset. Lewis goes out there and dumps a box full of fish before diving in to find her. They go to the beach and then takes her back home. Later, in the JuiceNet cafe, Charlotte tells him that she misses him and wants him back.
Lewis has all the typical weaknesses of a human.
Angus Mclaren did not appear in two episodes: "Happy Families" and "Just A Girl At Heart" (although he was mentioned). After the episode "To Have and To Hold Back" he left the show to film Packed to the Rafters, but returned in the series finale as a guest star.
In the final episode of season 3 "Graduation", Lewis returns to surprise Cleo. At Rikki's, when Cleo asks Lewis if he came all that way just to see her, he joked that he came to see the "new science genius in town" and "wanted to check out the competition." But hence him talking about Cleo. They kiss, which means they are maintaining a long distance relationship and now Cleo is as happy as can be.
- Lewis is really good at golf as seen in the episode "The Sorcerer's Apprentice."
- He played the drums when he was 10 and had lessons.
- Lewis won the junior film makers award three years in a row.
- His idea of a great date is fishing.
- His favorite food is burgers.
- He and Rikki shared the same taste in music.
- Main article: Lewis McCartney (JAW)/Gallery