Marrina is from an alien species known as Plodex. Her egg was found by Tom Smallwood, a fisherman in Newfoundland, who gave it to his wife Gladys. When Gladys broke the shell she imprinted Marrina with Human female DNA. Marrina grew up mostly normal, except for her odd appearance. She came to the attention of James MacDonald Hudson.
Marrina joined Department H when she was approximately 16 and was the first member of Gamma Flight. She quickly rose to Beta Flight and by the time that Department H's funding was cut she was ready to become a member of Alpha Flight. When Alpha became independent, she joined the team as an active member. She fell under the manipulations of the Master of the World and learned much of her history. She left Alpha Flight to explore a relationship with Namor. The two fell in love and married, after being a pawn in Atlantean politics and destroying her mate, a monster whose sole intention was to take over the world.
She and Namor eventually conceived children together, but she soon underwent a strange transformation, becoming Leviathan and attacking the Avengers. Namor had to slay her with the Black Knight's Ebony Blade.
However, Llan, the Sorcerer, told the Master of the World that Marrina was sleeping a false death under the sea. The Master eventually found her body and kept her prisoner in one of his bases, held in a stasis tube.
Marrina resurfaced in Norman Osborn's custody, and he used her as an endgame plan against Namor, who had recently betrayed him and allied with the X-Men. Osborn had her D.N.A. spliced and modified, making her a leviathan/snake-like being, driven by a hunger that could only be satiated with Atlantean blood. Once released, Marrina wrought havoc in the undersea kingdom, injuring many and killing even more. Namor and the X-Men devised a plan: using Magik and Pixie's powers of teleportation, they transported all of the Atlanteans across the globe leaving Namor as the sole bait. Once she approached the X-Men's new base, Utopia, Namor realized who she was. The Stepford Cuckoos scanned her mind revealing that there was nothing but rage, hunger and hate. Having no choice, they engaged her. Eventually Namor dragged a weakened Marrina underwater, where he performed a mercy killing, ending Marrina's life.
Amphibian: Although aquatic by nature, Marrina can exist as comfortably on land, as she does in the sea. She can remain underwater indefinitely, and function at ocean depths that would normally kill the average human being. Marrina possesses gills, which allows her to oxygenate her body, similar to that of a typical fish.
Shape-changing: At times, Marrina's Plodex genetics forced changes to her physiology.
Offensive secretion: Upon instinctual command, Marrina's body can secrete an oily residue designed to fend off attackers. The secretion causes the pupils of a victim's eyes to sharply constrict, rendering them temporarily blind.
Water control: Marrina has limited control over surrounding bodies of water. She can channel existing volumes of water into a large, moving spout, and ride upon the crest of the spout should she choose to. The exact limits and range of this ability have never been measured, but Marrina can direct the flow of water for several miles without any strain upon her person. In her first recorded mission as a member of Alpha Flight, Marrina channeled a spout of water for 3 miles, which she used to attack the Great Beast known as Tundra.
Swimming: Marrina is able to swim at tremendous speeds, rivaling that of even the Sub-Mariner. Her speed while swimming has been measured at 800 knots per hour, and it is believed that she might even be able to maintain speeds in excess of 900 knots per hour.
Leviathan: When Marrina's Plodex instincts took hold, she became savage and homicidal. Her hands transformed into talons that can cut through steel. After Marrina became pregnant with Namor's child, her Plodex genetics caused her to transform into a massive sea serpent over a mile in length, akin to the legendary Leviathan. It shot beams of kinetic energy from it's eyes, and laid thousand of Plodex eggs, using it's own devastation to "build a nest".
When Gladys Smallwood handled Marrina's egg, the unborn embryo biologically imprinted itself upon her. Using Gladys' form as a template, Marrina was born as a quasi-humanoid female. Traditionally, Plodex aliens assume a more formless, ambiguous shape.