What is the name of Arthur's best friend?

Arthur's best friend is Buster Baxter.

Buster is an anthropomorphic rabbit and one of Arthur's closest companions in Arthur. The two attend Lakewood Elementary School together and regularly share adventures, arguments, jokes, and lessons about friendship.

Buster is generally portrayed as outgoing, humorous, curious, and especially interested in food and travel. His parents are divorced, and his father, Bo Baxter, is a pilot. Buster's travels with his father became the focus of the spin-off series Postcards from Buster, in which he sends Arthur video postcards about the places and people he encounters.

Arthur has other important friends, including Francine Frensky, Muffy Crosswire, Binky Barnes, and the Brain, so the phrase “best friend” can sometimes seem broader in casual discussion. However, Buster Baxter is the specifically established best friend associated with Arthur's character biography.