User blog comment:Xilinoc/Ch. 612 - DIRTY/@comment-1338810-20150122165722/@comment-1268838-20150122222904

I think it's more likely that Rukia just knows her better than most other characters. How often have we seen Sui-Feng during downtime? Even at her worst, Sui-Feng's attitude has always been on a superficial level, during situations where she doesn't necessarily have any reason to be nice.