Fen was an Atlantean princess and the daughter of King Thakorr. When seismic disturbances caused by surface-dwellers' explosives threaten Atlantis, Thakorr sends Fen to investigate. She reaches the human icebreaker Oracle but begins suffocating after leaving the water. Captain Leonard McKenzie helps save her life, and the two eventually fall in love and marry.
Fen's relationship with Leonard creates the circumstances that lead to Namor's birth. When Thakorr sends Atlantean warriors to retrieve her, Leonard is killed and Fen is forced to return to Atlantis. She later gives birth to Namor and dies from a broken heart, leaving Namor with both Atlantean and human heritage. Her story is revealed to Namor by Ms. B and is the key to restoring his understanding of his identity.