Hanatori-san probably never had many friends or interacted with people that much if she thinks that Renako is plain. Spend 5 minutes speaking with her and you will see that she is anything but plain.
And plain looks? She definitely needs to take bath with her in order to fully enjoy her sexy body.
I didn't read that far yet into the novels but it is a news to me that Mai do not have a father. So he probably died young or got divorced, which does not suprise me in thr slightest giving Rene's personality.