I don't think she's real. I think she's symbolic of Zero's trauma, regret, and desired atonement for the child killings in the war. Although, in the flashback part of the ending we see that Zero actually didn't kill the child from his nightmare, and he refrains from killing the children in the bunker as well. The little girl could be symbolic of Zero not being a child killer. I imagine we'll find out more in the DLC.

It seems like Leviathan and Behemoth are representative of Zero and Fifteen, and that they're set up as two opposing symbols like Comedy and Tragedy, life and death. In the scene where Fifteen first appears on the highway, the same visual effect for Behemoth's glow when you find Leviathan appears from Zero. Add to that that in terms of the myth Leviathan is portrayed as a great sea serpent, a dragon, and I think there's a strong case for Behemoth representing Zero and Leviathan representing Fifteen.

With how the little girl is connected to Leviathan and Behemoth I think she's representative of the shared trauma of Fifteen and Zero in the war.

I think it's probable that the DLC will have us playing as Fifteen and I wouldn't be surprised if he does end up killing children in contrast to Zero. I think he is death and Zero is life. Or... Something like that.