Moving away from subjects heathen...
The Kingdom of Gododdin is perhaps the mightiest kingdom in the Old North. They are inheritors of the the legacies of the Votadini, of Coil Hen (Coel the Old), and his descendants Eidyn Adeiladu (Eidyn the builder), and Bran Godebog (Bran the Protector, also Bran the 3rd). Like most of the cumbrians, they follow a patronymic naming system, unlike their southern cousins who favor a tria nomina, though they do have a similar clan system. The most prominently Lwith are, to barrow the Rhegedian style, the MabEidyns, the MabBrans, and the MabGwrgi.
They have a deep seated rivalry with the Angles of Beornice to their south, but more recently Rhinnydd has become a thorn in their side. They get along fairly well with Rheged, all things considered.
As for Brythain, that's...complicated. If nothing, else, they respect their power. In the distant past they recognised the Auriadh as Comes Britainniarum, always with the understanding that their own king was the Dux Britanniarum. But when Bran Hen (Bran the First) died at the miraculous old age of 115, the then Âlt Rî, Andras, declared himself the sole Dux of Britannia. They were outraged and as a result, they refused to join his campaign against Beornice, allowing the angles to keep Andras South of the river Tees.
More recently, relations have warmed after they combined forces to drive King Ósric "Godsbane" out of Rheged. At the battle of
Urengwic in 746, Leûn and Custennin, princes of Brythain and Gododdin respectively, trapped Ósric between their two forces. The pagan army was destroyed. Since then, there have been talks between the two of them about unification.