Probably one of Adam and Eve's other unnamed children. The Bible frequently mentions men, but almost never women UNLESS essential to the story. We know Eve, but not most other wives. We know Noah and his sons, but only that their wives existed, not their names. Jacob had 12 sons. Only one daughter was mentioned later - because her rape almost started a war.
The geneologies of Jesus only mention Bathsheba and Mary - no other women, not even Eve.
It's likely that Adam and Eve had other sons and daughters, but they weren't considered important enough to mention specifically. Cain and Abel were mentioned because of the story of the first murder. Seth was the heir. Any others wouldn't have been important enough to mention.