At the beginning of the film, Jake Sully makes it clear in his monologue that Kiri was born from the body of Grace Augustine's avatar, which was empty. So, there is probably no father. Most likely, it is about some kind of immaculate conception. The deep connection between Kiri and Ava, which is described over the course of the film, speaks in favor of this version. James Cameron has previously flirted with religious themes in Avatar, for example by making the Na'vi blue like Hindu deities. It seems that some interpretation of the Christian myth is discussed in the second part.