She was the love interest of Allen.
Madison is a mermaid who twice saves her childhood love, Allen Bauer, from drowning. After kissing him, she dives into the sea and leaves Allen to return home.
Madison finds Al's wallet and shows it to the police. She later has a romantic relationship with Al.
However Madison is publically revealed to be a mermaid when she is splashed by Walter Kornbluth outside a restaurant and is summarily captured by scientists.
At first Al avoids her, in shock that his beloved was a mermaid, but later he rescues her from the lab with the help of his brother Freddie and Kornbluth himself, because the former helps him to realize that he still loves her.
They then escape the authorities by jumping into the bay, for mermaids have the special power to enable those close to them to breathe underwater.
Al joins Madison in the underwater world even though she tells him that he can never return to the human world.
However, years later, they find it necessary to return to dry land for a while, as Allen becomes bored on their deserted island hideaway and admits he misses life in New York City and his brother Freddie.