She sees Rikka as her master, whilst Rikka sees Sanae as her servant. According to them, this relationship was established in their "previous lives".
Sanae and Shinka have a very irritable, troublesome and sometimes even hostile relationship. Sanae thinks that Shinka is the fake Mori Summer, although Kumin thinks that she actually wants Shinka to be the real one and knows it. She always causes trouble for Shinka, enraging her leading to Shinka to make her eat dairy products. Despite all this, Sanae actually values Shinka and respects her.