The Source

Ramban
2:18

Hashem said, “It is not good for man to be alone. I will make a helper to face him.”

It is not good for man to be alone- It seems unlikely that man was created to be alone in the world incapable of begetting children, since all living things of all species were created male and female to have offspring. Herbs and trees, were also created bearing their own kinds of seeds. It is possible to explain man’s creation according to the view of R. Yirmeyah b. Eliezer who said: G-d created man originally with two profiles. A reproductive force or perhaps a seed, could pass from his male part to his female part as debated by scientists on the nature of pregnancy. The second profile of man helped the first profile to beget offspring. G-d saw that it would be good for the “helper” to face the man, so he could see it and separate or connect with it as he wishes. Thus, the verse says, I will make a helper to face him[meaning: I will make the helper who is now attached in the back of him, face him, unattached].

[Why was man as a single two part creation called not good?]

Man in his single two part state, could not be described with the phrase “it was good”, for he did not continue to exist in this form. The phrase “it was good” during the week of creation means it achieved its final form, as I have explained on the verse, And G-d saw that it was good. (1:10).