The reason why Satsuki doesn't see Renako as her best friend despite the latter insisting that it's their relationship is because they dated & even kiss each other before. Despite her lack of knowledge of romance & love, she doesn't that stuff should be done with friends, hence why she prefers to see Renako as her ex-girlfriend.

>>280321743
About this Youko girl. Satsuki does know that Mai's mom gave her a mission to break Mai & Renako up, right? Why does Satsuki keep trying to stop her when she's currently trying to do the same thing?