Read the following passage and answer the question that follows.
"His instrument struck against something hard, dangerously near the kidney.... 'It is not quite at the kidney, my friend,' Sadao murmured.... 'My friend,' he always called his patients and so he did now, forgetting that this was his enemy."
To whom does Sadao attend to in the lines above?
Step 1: In the passage, Sadao calls the person "my friend," which indicates that he is addressing this person in the manner he typically uses for his patients. However, it is also mentioned that Sadao forgets that this person is his enemy.
Step 2: Based on the context, it is clear that the person Sadao is attending to is not his friend, but his enemy, since the passage highlights that Sadao forgets this fact momentarily. Thus, the answer is \( \boxed{{C. His enemy}} \).