User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-1359322-20131221155510/@comment-16281343-20131221163002

Can't find notable women to be a candidate for Ross' grandmother except Brynhild (which is a Valkyrie, possibility of her being Ross' grandma is low) and the Norns.

~Btw, I found this while scouring the net about Norse characters.

"Vali: Odin's son: Ragnarok survivor."

So, Vali being Odin's adopted son is not just Ishibumi's whim. :D