loner
n.
"one who avoids company," 1946; see lone. Apparently first in U.S. baseball slang (earliest reference is to Ted Williams).
Ted 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]