According to Norse myth (which, I'm keen to stress, deviates from my own records), Eysa was the daughter of Glod. My research instead places them as siblings, both fathered by Surtr, but born to different mothers.
Eysa is one of Muspelheim's most elite warriors, and effectively a princess to her people, albeit one who could tear heads from shoulders. Her role in the Svartalfheim conquest is unclear, but there is speculation that she may not fully agree with her father's methods.