What was the name of Mark Twain's friend and fellow realist writer?

Mark Twain’s friend and fellow realist writer was William Dean Howells.

Howells was an American novelist, critic, playwright, and editor who became one of the leading advocates of literary realism in the United States. His best-known novels include A Modern Instance and The Rise of Silas Lapham, which examine ordinary social life rather than romanticized adventure.

Twain met Howells in 1869, beginning a close friendship and literary correspondence that lasted for decades. Howells later helped promote Twain’s work and offered editorial support and criticism. Twain regarded him as an exceptionally important writer and praised his command of English-language literature.

The question can cause confusion because Twain also knew Henry James and Charles Dudley Warner. Warner was Twain’s collaborator on The Gilded Age, while James was a fellow major novelist. Howells, however, is the specific friend identified with Twain’s long-running literary partnership and American realism.