loner (n.)

"one who avoids company," 1946; see lone. Apparently first in U.S. baseball slang:

Ted [Williams] is likable enough in spite of his obsession with his specialty. He is something of a "loner," and he refuses to pal around with his teammates in off hours, but in the clubhouse he does his share of the talking. [Life magazine, Sept. 23, 1946]