Betty Dean is one of the oldest supporting characters in the Marvel Universe, having appeared in publications for over 70 years. Below is an abridged version of her history. For an unabridged version see the expanded history of Betty Dean.
When the Atlantean Emperor, Thakorr, sent Namor (Sub-Mariner) to wreak havoc on the surface world, Betty Dean was selected by her police chief for a dangerous mission to apprehend him. Betty was eager to stop the Avenging Son on her own, and in his own element.
A former swimming champion in college, Betty feigned that she was drowning to attract Namor's attention. Namor had never seen a surface-woman before, and was successfully distracted by her beauty, when she pulled a gun on him and demanded that he peaceably return with her. They were interrupted when Nazi U-boats began attacking a boat. Namor took Betty to the safety of a nearby island and went to investigate (ultimately defeating the Nazi sub and saving the ship's crew). Betty explained to Namor that Americans aren’t evil, and although he wasn't fully convinced, he agreed to destroy the Nazi blockade and keep warships out of his waters.
Namor respected Betty for her courage and love of adventure, and the two remained friends for many years (despite their differing views of the surface world). There was an underlying romantic tension between the two, and it is suspected that Namor may have been attracted to her for qualities that reminded him of his mother, Princess Fen. However, Namor and Betty were never officially an item and because of Namor’s slower aging rate, the two eventually parted ways.
When Namor was reuinted with Namorita, he soon decided that his young cousin (bereft of parents and an undersea homeland) needed a proper guardian at this important age of growth. Despite Nita’s protest, Namor left her to Betty and reassured her that Betty was a shining beacon of warmth, kindness and strength. Under Betty's guidance, Nita attended high school and college. The two became very close and Nita eventually considered Betty to be her second mother (and took her last name when she died).
In 1983, Dr. Herman Frayne (Dr. Hydro) decided to conquer Atlantis so he began hijacking airplanes and forcing them to land on Hydrobase. Once there, the passengers were taken into a chamber, where Hydro exposed them to the Terrigen derivative, mutating them into amphibians (of which there was no known cure at the time). One of the hijacked a planes carried Namorita and Betty. Namorita was unaffected by the gas, but Betty was transformed into one of the amphibious Hydro-Men.
Hydro attacked Atlantis but he was defeated and killed. Unfortunately, the Atlateans were rendered comatose by a nerve gas during the fight. While Namor tried to find a cure for the Atlanteans, the villains Tiger Shark, Attuma, and Dr. Dorcas launched an assault on Hydrobase and imprisoned Namor in a dehydation chamber. Dr. Doom came to the rescue (at this time he was an ally with Namor) and teamed-up with the newly amphibious Betty. During the resuce, Dorcas attempted to kill Namor, but Betty jumped in front of the blast and was herself killed.