The Bible didn't mention a name but logically, Cain married his sister. I mean think about it, God created only Adam and Eve and they gave birth to Cain and Abel. Cain killed Abel and was cursed to wander the earth after which he slept with someone that bore him a son named Enoch.
It's obvious that Cain didn't have any sexual relations with his mother Eve as she was with Adam throughout the Genesis chapter.
There must have been a girl borne of Adam and Eve that wasn't mentioned because as written in the Bible the earth was void of humans before God created them and the creation story follows only Adam, Eve and their family as they were the only humans created according to the Bible.
Due to the several translations of the Bible, some information were lost due to disasters and inability to decipher some ancient words, these I believe are the reason why the identity of Cain's wife and all other unanswered questions weren't mentioned in the Bible.
Do have a happy Sunday !!!
I answered a similar question recently.
Cain married a female from a nearby town.
If you read Genesis you may notice that there are two accounts of creation.
One where a Male and a Female are created as equals at the same time, and another where a Male is created first and a Female from his rib and made subject to him.
In Jewish tradition, the first mate of Adam was Lilith, his equal, but they broke up and Lilith left the Garden of Eden.
It is safe to assume, between their break-up, creation of Eve, and Adam and Eve being banished from the Garden of Eden, enough time passed for children of Lilith [presumably conceived together with fallen angels or demons or giants] to create a society Adam, Eve and their children were welcomed into later.
Hmmm! Well, I am sure most people will want to argue this, but the truth is, there was a first and second creation.
The first one was where God created human beings both male and female with a word of mouth.
Genesis 1:26-30. These verses will definitely answer your question
The second creation was when God Himself created a man called man. He formed Adam from the soil and breathed life-giving breath into his nostril and he began to live.
Adam and Eve weren't the only humans on earth after creation, but they were the only ones that were placed in the garden of Eden and had a personal relationship with God. They communicated with God most of the time.
This question really baffles me, as we all know the first man and woman on earth are Adam and eve and they gave birth to Cain and Abel, in which Abel was killed by his brother, so I also wonder who Cain got married to because it wasn't stated in the bible