As everyone whose played Blazing Sword would know, Nergal's main drive in the story is power, starting with his wife Aenir's kidnapping.
Nergal must have felt guilt over not being able to save Aenir. And after his fall into darkness, he began to create morphs to further his goals of now obtaining access to the Dragon's Gate, especially his strongest morph, Limstella, who acted as a collector of Quintessence for Nergal.
Limstella, in Nergal's words, possesses his ideal strength and beauty. Her beauty could be one that Nergal had encountered before; he could have thought the same for Aenir. The power Limstella posesses (Fimbulvetr) might be Nergal's fruits from his obsession and search for power. Nergal had even given her the ability to feel emotions, suggesting that he designed Limstella with something special in mind. Maybe companionship?
As a closer, I also drew up a comparison of Limstella and Ninian in the latter and former's palate.
Notice how similar their eyes are.