Weeeelll, as DearHeart did EdWin already, and then Royai was done too, this leaves me with Ling/Ran Fan.
In many ways, Ling and Ran Fan are a direct parallel of Roy and Riza. Ran Fan's largest drive is to propel Ling forward into ruling over their land; both for his sake, and the sake of the people.
Ling is charismatic, determined, goofy, and level headed. Ran Fan in contrast is quiet, serious, and shy, but also equally determined.
She is utterly devoted to him, and, though shown more subtly, he is just as devoted to her.
She was willing to cut off her own arm so that he could go forward; she did not kill herself, but cut off her arm so that she could live, trick the homuncli, and be able to help Ling. She pushes her body to its limit in the automail rehabilitation process so that she can fight for and protect him.
Words against Ling effect her the way personal insults would not; she becomes furious, and fights even harder.
Ling on the other hand is her lord. Her master. She is supposed to be loyal to him; she has been raised to be nothing else. Ling has no reason to be devoted to her on the same level. And yet, he is furious when Bradley refers to her as 'baggage'. He will not let her be left behind in the sewers after she cuts off her arm (and, indeed, his actions when we see him after she did are all full of anger, but also very very fast- he wanted to get to her quickly.) After he becomes Greed, he makes sure Ed and Al get a note to Ran Fan about the philosopher's stone. His face when Envy turns into her; like with Hughes, it is beyond surprise. He simply is unable to hurt Envy when he looks like Ran Fan.
It is Ed shouting her name that brings Ling forward from Greed's control, if only for a moment. He tells Greed it is because he has to be able to face Ran Fan that he must become a homunculus.
They also bring out the best in each other.
Around Ling, Ran Fan is shy without her mask. She is quiet, and always does as she is asked. She views herself as a servant.
However, Ling treats her as a person, as a friend.
Ran Fan, (and Fuu) are his family. He states in the manga that he doesnt really know his father, and Ran Fan's unwavering loyalty and trust and encouragement are more than enough to make up for his lack of a family.
Toward the end of the manga, after Fuu's death, Ling tearfully declares that he had wanted to share immortality with her and Fuu. With his family.
The relationship between Ling and Ran Fan is about loyalty and devotion, and family. Ling treats Ran Fan with the respect she has not been raised to get, while Ran Fan gives him the support no one in his biological family did.
And, though this isn't something either of them actually did, when faced with a dying Wrath, the homunculus tells her that "No last words are needed between a king and his partner." That he said this to Ran Fan of all people seems very telling to me, as if he knew she would understand it.
Song: Orchard of Mines, by Globus
www.youtube.com/watch?v=e8jmOYnedk8