Zoro x Luffy
Zoro understood the first day he met Luffy that he would never be able to leave the boys side; that no matter what he did, he'd always remember the Straw Pirate. Zoro understood that, and even if he wasn't too pleased with the thought at first, after Luffy had smiled and taught Zoro how he saw things, Zoro was content with the other.
When others joined, he didn't really mind. In fact, he didn't care much for them, because they were all just lower ranked than him. He was Luffy's first true member of their pirate gang, and no one could change that. Of course, maybe he couldn't cook as well as Sanji, or couldn't navigate like Nami – but none of that mattered, because he was Luffy's first member and only samurai.
Luffy told him one night, a few nights before they would reach a port and the moon was full and bright, the starts out-shined by the moon's ethereal beauty, his true feelings: that no matter where Luffy went, far or close, he would always feel Zoro with him. That he could feel everyone in his heart, but the one who rang the loudest was Zoro.
That night, Zoro decided he would always, always, protect Luffy. If only for the fact Luffy would hold him that much closer to his heart.