who would i look up to?

I don't really have that much to say relating to their inevitable friendship, as I think it's fairly obvious that during the whole Soul Society arc, Renji is acknowledged by Byakuya (a feat in itself) as someone he should respect. This alone is a mindset that could build a closer, positive reationship between the two of them and lead to further trust between the two shinigami.

In chapter 181 - the infamous recovery scene - Renji states that he is glad that Byakuya didn't die, as he would not have anyone to look up to if the nobleman did, in fact, cease to exist. This is both the word of an admiring subordinate and, I think, the beginnings of a relationship based on mutual growth rather than Renji simply competing with him. They are comrades now rather than enemies, and if one takes the definition of friendship quite literally (a relationship in which those involved will seek out each other's company and exhibit mutually helping behavior), they are friends.

This is also especially obvious when, in chapter 247, it is noted that Byakuya helped send Rukia and Renji back to Hueco Mundo. While part of this relates to helping Ichigo and the cause at hand, I doubt Byakuya would have involved himself if he did not see Renji as capable and worthy of his assistance.

Renji's nature will eventually allow Byakuya to heal, especially considering Renji's attachment to Rukia. If Renji can forgive his captain for the conflictions of his past, eventually, Byakuya will be able to forgive himself. In turn, if Byakuya continues to acknowledge Renji's presence, Renji will more than likely have a better opinion of himself - poor puppy.

I think this is a fairly obvious part of their developing relationship, so this is about all I can say on it. For some complex, mind-bending analyzation, continue on to my ramblings on how these two could eventually be lovers.