So I got tired of reading Ichiruki fics where Renji is portrayed as some poor, lovesick fool and decided to write a fic depicting what I believe to be Renji's feelings for Rukia. It turned out shorter than I intended, but I think I hit most of the points I wanted to get at.
Semantics
The general consensus was that he loved her.
It was certainly true. He wouldn't deny it. He did love her. He loved Kuchiki Rukia more than he loved anyone else, living or dead.
But the common misconception was that he loved her in that way. You know, the romantic, I want to be with you forever kind of way. The way that a man loves a woman such that he would do anything for her: protect, fight, even die.
He supposed he couldn't blame them. The way he acted did tend to fit that mould. He would do anything for Rukia, go to any lengths to ensure her safety.
However, the simple fact of the matter was he didn't love her like that. Or at least, he wasn't aware of it. You see, he wasn't the kind of man who thought deeply about things like this. He didn't do that introspective, look inside your heart shit. That was for souls that had time to kill. That wasn't him. If he had extra time to lay around and think, he might as well spend it trying to get stronger. Because you couldn't protect anything if you were weak.
To him, there were two kinds of souls: the kind (usually without reiatsu) that dawdled in Soul Society until they died and got reincarnated, and the kind (usually with reiatsu) that trained to get stronger to protect something.
He was the second kind. And he wanted to protect two things. Like all good shinigami, he wanted to protect Soul Society and defenceless souls against Hollows. But first and foremost, as a person, he wanted to protect Rukia.
Because he loved her.
Because she was the single most important person to him in the world.
Because she was his light in a dark world. She was the reason he lived.
Because while he had other friends now, she would always be the one he considered family. The others had died back in Rukongai, leaving only Rukia and himself.
So you see, he did love Rukia, but that love didn't have the label everyone seemed to attach to it.
Maybe everyone was right and he loved her like a man loves a woman.
Maybe everyone was wrong and he simply loved her as a friend.
Maybe he loved her like a sister—although that would lead to unfortunate comparisons to Byakuya, her actual brother.
Who knew? Even he didn't for sure. And he felt no reason to define it. He loved Rukia and he wanted to protect her. That was enough for him. Let everyone else draw their own conclusions.