Rukia/Renji makes a fair bit of sense.
They grew up together but I'll admit this means nothing when push comes to shove. What makes it relevant is that they were united by their hardships and it indicates that they have multiple similarities.
They're parting seemed to be hard on both of them. Rukia felt Renji was growing distant from her because he was growing instrength at an astounding rate and then Renji thought Rukia was growing distant from him because she got adopted by Byakyuya. Renji had absolutely nothing against Byakyuya until he took Rukia in and now he's commited to surpassing him because he feels he'll be on Rukia's level again. This is why he asked Ikkaku to train him, it's why he achieved bankai and it's why he was willing to disobey orders from his superiors. All of it for Rukia.
The question is that now he and Rukia seem to be chummy again he doesn't make his move. It could be he still thinks he's unworthy and so will only make his move once he's a captain(it's practicaly preordained he'll be one)or it could be that Rukia's reaction to Ichigo's arrival to rescue her scared him off and so he's waiting to see how it plays out before he makes his move.