Roaring Moon and Iron Valiant are my two favorite Paradox Pokemon, and these two are direct counterparts among them and share the same distinct standout properties as the "final boss" Paradox mons of the lot, having 590 BST, being among the legendaries in the Pokedex order, and whatnot. They are also the most different since Roaring Moon draws more from
Mega Salamence while Iron Valiant is a hybrid of Gallade and Gardevoir (and also draws from their Megas to an extent).
But what's pretty damn cool is that the basis of these two works well with respect to each other flavor wise, and one thing I like about this duo is that together they (whether intentionally or not) end up embodying the dichotomy of dragon vs knight: the bestial, dreadful dragon and the warrior knight who serves to strike it down, because of the species they derive from.
Iron Valiant is a hybrid of Gardevoir and Gallade, who are both perhaps the most "knightly", "chivalrous" Pokemon in existence. Gardevoir's name in Japanese even translates to "Sir Knight". It is in lore a loyal, devoted, guardian of a Trainer is shares a close bond with, it will protect its Trainer with its life, true to how an ideal knight in fiction would act. PLA's Dex entry (from Laventon's POV) even describes its "dress" as akin to that of nobility. Gallade is a more obviously designed warrior, it is a swordsman with blades for arms. It is attuned to the wishes of others for help, it will rush in to guard those in need of protecting. And it is a masterful sword fighter but also very honorable and courteous in combat, true to the ideal image of a knight, albeit in a different fashion from Gardevoir.
Roaring Moon is more based on Salamence (albeit Mega Salamence), who is basically the embodiment of the textbook European bestial dragon you often see in folklore. There isn't much to say about it, but among the many Dragon-types we had Salamence is basically standout for being the textbook bestial monster that we often think of European dragons as. Its physique is built like one, it can fly, it breathes fire, and it's aggressive and often destroys its enemies with its breath of fire and shredding claws.
Their respective dual typings complement each other well in this regard, with the knight being the one who slays the Dragon. Iron Valiant's dual typing is a hybrid of both Gallade and Gardevoir's second typings (Fairy and Fighting), but both of these types also happen to be advantageous against Roaring Moon's type combination, which is Dragon and Dark. Fairy is the type that slays Dragon, while Fighting is advantageous against Dark with Fighting type representing the honorable warrior vs Dark being "Evil-type" in Japanese and the type representing either dishonorable/dirty tactics or sheer malice. Hence Iron Valiant having a total advantage against Roaring Moon: Iron Valiant being derivative of a pair of knightly/chivalrous mons while Roaring Moon is a derivative of the textbook bestial monster dragon.
Many Paradox counterpart duos come off as rather arbitrary as to who is a counterpart to who, aside from the Volcaronas and Donphans, but this one, being the "special" pair of Paradoxes above the rest barring the cover legends, is a clever pair as to Valiant and Moon in particular being counterparts with each other, in a pretty fun way imo.
(Yes, for the record I know all of the Paradoxes in universe are depicted as aggressive and cruel, but that can be attributed to them being not of this world and being in a world that they don't belong in, similar to the Ultra Beasts, who are also extremely aggressive to everything in the normal world, and despite the differences both are foreign to the normal Pokemon world and are likely on guard attacking things that are unfamiliar to them as they live in a world where they don't belong).