Tess Samuels is the older sister of Amy, the leader of the mermaid group. She is often crude and rude to Amy, and doesn't care much for her. One day, Kelsey made a scrapbook full of pictures of the girls as mermaids, but it unfortunately falls into the ocean. While at the dog park, Tess finds it in the water, and brings it home. Amy sees it, and freaks out, telling her not to open it. Amy's mentioning of the scrapbook makes Tess curious, and opens it, finding all the pictures.
Later, the Amy and Brenna decide to tell Tess, but Kelsey is hesitant. Happy, Tess takes pictures of their tails and their powers in action. Brenna thinks its just for fun, but Kelsey is yelling, saying how it could put their secret at risk.
It turns out, Tess was gathering the pictures to send to a marine lab in Key Largo. When the girls find out, they try to run after Tess in her car, but it was too late; she was already driving to Key Largo.
Tess gives the package to the lab, and leaves. Later, while Tess was gone, the girls snuck into the lab with their friend, Blaire, and stole the package. In season five, it shows that Tess was arguing with her mom, who was telling her that she was crazy. Tess exclaimed that she wasn't kidding, and in the end, her mom decides to send her to a school for 'troubled teens'.
She later returns in Christmas time, bright and cheery, saying that she didn't believe any of the mermaid stuff anymore, that the school put it in her mind that she imagined it all. Kelsey and Brenna were convinced that Tess made a sincere change, but Amy thought that this was all a plan, referring to the girls how Cruella Deville in 101 Dalmations pretending to be nice, just to steal puppies; like Tess pretending to be nice just to expose their secret again. But it turns out, Tess DID change.
Since it was cold, Brenna and Kelsey were complaining how they couldn't do what they loved most: swmimming. In the end, they all decided to go in Amy's jacuzzi. Tess got back from grocery shopping, and looked around for the girls. She found them in the back, shocked to see their mermaid tails.
Later, she was grumbling to herself how they said it was all her imagination, but it was real. She said how she was going to "show them all".