One of the few characters who retains his real name in the novel (like Annushka-the-plague). V. I. Kuz'min treated Bulgakov for his nephritis in the 1930s.
Elena Sergeevna, Bulgakov's wife, lived in the house described as in the novel as Kuz'min's. (The buildings were torn down when the Hotel Pekin was built.) The real Kuz'min actually lived not far away, at Sadovo-Kudrinskaia 28.