I'm incline to think it is his mother.

The possibility of Nael desu Darnus being the result of Naels dying thoughts was a possibility mentioned directly by Alisaie (or was it Urianger later?), and after regaining his senses, Nael makes a passing mention that he was once a weak child. He suggests that he had a fairly traumatic childhood, considering that he only even brought the topic up with regards to the "trauma" Alisaie would go through continuing her quest. Something terrible happening to a beloved elder? I'm incline to think Nael had a good relationship with his mother, as a weak child one would certain seek their mother for comfort, and at deaths door I'd imagine he'd think back to her (he was once again "weak") and confuse Bahamut. Can't imagine babby Nael would confide with his father, considering what is suggested he did to his father, if anything I'd imagine his father did something to his mother, hence the bloodshed and heartlessness...

I really kind of hope they expand on the White Ravens history at a later date, there is a fair bit more they could do with the character, even if he is dead. Heck, Gaius could also do with some fleshing out, considering right now I get the distinct impression that he was basically crucified in order to rebuild the realm... I just found him to be so very badly written in 2.0... At the very least I'd like to know what drove him to such an obsession with conquest... Nael we know got corrupted by Bahamut, but Gaius? He just ran off on a failed conquest and left the Emperor to die, although I'm starting to doubt that Gaius wielding Heirsbane was in anyway relevant, considering the weapon seems to be standard issue now (here's a fun game, how many copies of Heirsbane did you spot in Azys Lla?).