Adam and Eve had two children Cain and Abel. Cain killed Abel. There is no knowledge about a female companion for Cain(male) in the Bible. Genesis 4:17 Cain had a son Enoch. Who is Enoch's mother? How is Enoch born?
Interesting question indeed ! I made some researches on wol.jw.org, and came accross this reference : http://wol.jw.org/en/wol/d/r1/lp-e/2010649
Here is the summary :
Although this is often asked as a trick question by Bible skeptics, the Bible does provide sufficient detail to give a satisfactory answer.
Genesis chapters 3 and 4 present the following information: (1) Eve was “the mother of everyone living.” (2) Time elapsed between the birth of Cain and his offering the sacrifice that was rejected by God. (3) Following his banishment to become “a wanderer and a fugitive,” Cain worried that ‘anyone finding him’ might try to kill him. (4) God set up a sign to protect Cain, indicating that either his siblings or other relatives might try to kill him. (5) “Afterward,” Cain had intercourse with his wife in “the land of Fugitiveness.”—Genesis 3:20; 4:3, 12, 14-17.
From the above, we can rightly conclude that Cain’s wife was a descendant of Eve born on an unknown date. Genesis 5:4 acknowledges that during his 930 years of life, Adam “became father to sons and daughters.” Of course, the Bible does not specify that Cain’s wife was Eve’s daughter. Indeed, the fact that she is mentioned after Cain’s banishment indicates that enough time had passed that she could even have been one of Adam and Eve’s granddaughters. Hence, The Amplified Old Testament describes Cain’s wife simply as “one of Adam’s offspring.”