First Appearance: Marvel Mystery Comics #82 (May 1947).
Golden Age Appearances: Marvel Mystery Comics #82-88, Sub-Mariner Comics #23-24, 27-28, 33-42, 36, 39-41, Human Torch #31-32, Captain America Comics #68, 70, Namora #1-3.
Modern Appearances: Sub-Mariner #33, 50, 51, 61 New Warriors Annual #1, Namor #20, What If v1 #9, Marvel: The Lost Generation #2, Citizen V & the V-Battalion v2 #1.
Years Active: 1947-
During the Golden Age Namor the Sub-Mariner was not quite the lone wolf he's become in the modern era. In fact, he had something of a family, including a cousin, Aquaria Nautica Neptuna. At one point Namor, having successfully concluded a case in Asia, returns home to Atlantis, to find that everyone in Atlantis has been wiped out by a gang of criminals in a submarine. Everyone, that is, except Aquaria Nautica Neptuna. Namor nurses Aquaria back to health, and when she's up and about, she and Namor jointly swear vengeance on the murderers of their race. To emphasize this point Aquaria changes her name to "Namora," which in Atlantean means "avenging daughter." From that point on Namora had various adventures, both with Namor and on her own. She even teamed up with the Golden Girl (I) and Sun Girl.
In the modern era Namora was brought back and her past fleshed out. She is/was still Namor's cousin, but was the daughter of a human mother and an Atlantean father. She was born in Maritanis, which is where Namor discovered her; it was Maritanis which was destroyed, not Atlantis, and the gang of criminals used nuclear torpedoes to destroy the city. In 1953 she surfaced long enough to attend the funeral of the Union Jack (II).
After Namor's disappearance in the 1950s Namora married Talan, an Atlantean, and the pair emigrated to the Lemurian city of Lemuria. It was in Lemuria that Namora was killed by the Lemurian princess Llyra. At some point before her death the New Warrior Namorita was cloned from her by the Atlantean geneticist Vyrra (the creator of Mako, of the Lost Generation). At another point before her death she helped the Monster Hunters.
Namora's powers were quite similar to Namor's; she was very strong, could swim quite quickly, and could survive underwater indefinitely--the whole Atlantean package, essentially.
Useful Namora Site
Namora. The page from whence I stole the above image.