Things that are different can clash. They can differ far too much for there to be any room for acceptance or tolerance. But opposites can also attract.
Ivan is large and broad. He has a certain lack of grace and doesn't have a subtle presence because of all these facts. Although something about the bulky frame doesn't suit the softness of amethyst depths, the quietness that accompanies him. He speaks when spoken to, Russian tones usually gentle and polite. He rarely says a word and yet he can't be ignored; he towers over most purely in size.
Arthur is trim and smaller. Not scrawny in any way, but compact and someone who stands on the toes of others to crane his neck into view. And yet at the same time he demands focus. He speaks out for himself fiercely and against what others speak. Although petit he will hold his head high, a proud spirit singing out and so he in turn cannot be overlooked.
Those are differences that fit around each other very well, and they compensate for what the other doesn't have. When Ivan can't find the words for himself Arthur is there to assert him, and Ivan can be a quiet comfort when Arthur has grown tired of keeping face in front of the other squawking nations. In some senses they complete each other.
But the differences are not what brought them together or what they see as the foundation of their relations. They are both old, betrayed men, both imperfect and both hated and mocked by others in their eyes. They have tension and rivalry in their past along with alliances, so they are able to see the good and evil in each other and in their history.
They came to see each other as equals, as men even if not as countries, and that was when they began to believe that two solitary people could find a serene happiness, that splendid isolation could become a more enticing togetherness.