Shunsui Kyouraku
your best drinking friend
Mr. Captain of Division 8 is like a twin image of Rangiku. Both love to drink, both love to skip paperwork and both are trustworthy leaders who don't like to show off their true power unless situation calls for it. Due to Shunsui's easy-going personality, he's made a handful of Soul Reaper friends equal to or below his rank, among which Rangiku takes a significant part.
Whether it be their similar outlook in life or a common love for fun, Shunsui is without a doubt, a very good friend of Rangiku's as confirmed by Kubo himself. The two would take a cup together whenever possible, or maybe share little tricks on how to run more effectively away from their respective "watchers" - Hitsugaya and Nanao to be exact. As far as speculation goes, I would not be surprised if Rangiku drags Shunsui to help with her SWA tasks from time to time. Captain or no captain, the two are probably the closest to each other in Soul Society next to their own partners and in Rangiku's case, a family to match.