Who is the character John Wick's doctor, and how does she fit into the John Wick universe?

John Wick's doctor is a character named Dr. Marta, who is played by the actress Anjelica Huston in the movie John Wick: Chapter 3 – Parabellum.

Dr. Marta is a prominent figure in the world of assassins, as she runs a ballet school in Morocco that serves as a front for a refuge for assassins. In the movie, John Wick seeks out Dr. Marta's help after he is declared excommunicado by the High Table, and is forced to flee New York City.

Dr. Marta uses her medical knowledge to help John Wick recover from his injuries sustained during his escape, but only agrees to help him after he swears a blood oath of loyalty to her. This oath is an important part of the John Wick universe, and carries significant weight among the world's assassins.

Dr. Marta's character adds an interesting dimension to the John Wick universe, as she is one of the few individuals that John Wick trusts, and is willing to work with. Her role in the movie also sets up potential storylines for future movies in the franchise.

Overall, Dr. Marta is an intriguing character in the John Wick universe, and her role as John Wick's doctor and ally is an important part of the story.