The general consensus is that Twain did not write "nigger"
as part of Jim's proper name.  Albert Bigelow Paine does use it
as such in _Mark Twain: A Biography_ when he states:  "The tale
of Huck and Nigger Jim drifting down the mighty river..." (p. 794).

Given Twain's attention to details of capitalization and punctuation,
the argument that the phrase "the nigger" was intended as a part of
Jim's proper name in _Huck Finn and Tom Sawyer Among the Indians_
is weak.