Who is Polygraph Polygraphovich Sharikov from Heart of a Dog and what is their importance?

After the surgery in which Klim's pituitary gland and seminal vesicles are transplanted into Sharik's body, Sharikov is tranformed into a creature that is a combination of a dog's body with a human brain. Unfortunately, Klim takes over Sharik's personality. After the surgery, Sharik's first-person narrative disappears, an Klim's personality dominates and eventually completely takes over. Philippovich eventually reverses the surgery, and Sharik returns to normal.