Diana is the oldest of 3 Mermaid sisters in a movie Mermaids. Diana is portrayed by the actress Erika Heynatz.
Diana tries to stop fisherman John Mallick and his assistant Carlo from taking her father whom they found in the sea killed by the fishing bombs they used. She slams the vessel with a powerful force. Mallick looks into the water and sees a figure surface, pointing a trident at him, and he escapes.
Diana is a beautiful but fierce mermaid with superhuman strength, and a magical trident that turns into a comb she wears it in her hair. Although she loathes human beings, Diana ventures on to land for the first time to seek out her two sisters, Venus and June, to help her find Mallick and seek justice for their father's murder.
Diana finds out that Venus works at the bar as a mermaid act due to it's owner, Earl, having stolen Venus' Birth Right, a pearl tiara. Diana threatens Earl until he agrees to give up Venus' tiara, but she also finds a large seashell, apparently the Birth Right of another mermaid. The shell belongs to Earl's middle-aged wife, Betty, who has forgotten her mermaid heritage after spending so much time on land. Venus gives the Birth Right back to Betty, but she wants to stay on land with her human daughter.
The three sisters then see on the front page of a newspaper that Mallick has discovered the body of their father on the seabed, and is keeping it in a freezer. They confront him, but Mallick has researched mermaids, and knows that they are bound to grant one wish to anyone who asks for it. He tells them to find him a chest of sunken treasure, to which they agree. After they bring him the gold, however, Mallick reveals that he wants to use the three sisters as the main attractions in a theme park. But his plans are ruined when the police arrive at his warehouse, following Randy's tip-off. When the police check the freezer, the merman's tail has transformed back into legs as the freezer is dry. They arrest Mallick on suspicion of murder, despite his protests that the three women are mermaids and the body is a merman.
At the end Diana says that she will stay on land as well, so that she can watch over her younger sisters as their father would have wanted.