So Nerthus (I will be using the modern spelling because I don't even know how to type how FEH spelled it), is a goddess commonly associated with Njordr. She is the Norse Vanir goddess of fertility and was worshipped through a ritualistic cart procession, and may be the unnamed sister-wife of Njörðr, the father of Freyr and Freyja. Interestingly her old Norse name roughly translates to 'The Contented Powerful One' or 'The Contented Paradise' depending on if use the Old Irish or Old English entomology. Both Nerthus and Njordr also have a link to the obscure Norse goddess Njorun, another contender to be Njordr's mystery sister wife.
As for in FEH, from the bit of dialogue and preview of a battle we saw she doesn't seem to like Njordr and our heroes by association. If she is indeed his wife, or rather former wife, in this universe then their falling out could be related to how their children turned out or even the Golden Curse.