Air
Rukia doesn't walk, Renji knows. She glides--he knows it sounds stupid but it's true--she slides on the ground with her feet and you don't know she's walking, she looks like she's sliding gleefully, childlike on ice, or perhaps on certain days tiptoeing, an air sprite on the CO2 emissions surrounding her. Damn humans. This is why he comes to take her back. She doesn't belong there, her feet don't stay on the ground--she's just asking for trouble, he knows, you can't make friends with a fairy.
Renji doesn't notice his own solid tread, crunching leaves on the ground. Shinigami are still human, in almost every sense of the word.