Recalls are individuals reborn (as infants) into the world from a past life via the wishes of the world itself; they are gifts from Mnemosyne. They are biologically the same as their old selves, but are otherwise left to the world to be raised. It is possible that they will be absolutely nothing like their past selves, and the ones who wished for them may not ever see them.
The frequency of Recalls is not all too rare; about 1 in 20 individuals is a Recall. Their survival rate is a bit lower than a regular being's, though - as Recalls are born Chaotic (being from Mnemosyne and all), they face higher risk of not only being devoured by that Chaos, but being victims of the Night, who are drawn to Chaos.