Household Rank
Where a household keeps several slaves, one of them is the senpei. She is whoever the owner has put there — usually the oldest or longest-serving, because experience is what the position needs, though the choice is entirely his. She trains the others, answers for the work, and gives instructions that hold; her word stands in for his, because she knows what he wants better than any other slave in the house and was chosen for exactly that.
A junior slave who thinks she is wrong does it anyway. Going over her head to the owner questions his judgment about who should be senpei, which is a larger thing than whatever the disagreement was; if she is wrong it is hers to answer for. A junior addresses her as Senpei; some would rather be called by name and say so, and an owner does not involve himself in what one slave calls another. The senpei corrects the slaves under her with words; anything physical belongs to the owner alone. When no physician is there and the owner is away, the decision may fall to her.