M. Yoshoto is the director of Les Amis Des Vieux Maitres, ("Friends of the Old Masters"), a correspondence art school in Montreal, Canada. He is originally from Tokyo, Japan.
Salinger only discusses World War II indirectly. One such indirect reference comes in describing M. Yoshoto. The dramatic irony (where we something the character doesn't know or isn't talking about) is strong here. When Jean evokes the character Fu Manchu, he evokes the harmful stereotypes of Asian people, and anti-Asian sentiment that were so common in World War II.
Nineteen-year-old Jean seems to be oblivious to this problem, and finds M. Yoshoto "inscrutable" just like the character of Fu Manchu (20). ("Inscrutable" here means mysterious.) A common stereotype of those days is that Asians were mysterious, in order to cover up their difference from Western people, and their nefarious plans.)
Salinger may have expected us to view the Yoshotos as refugees from Japan. Quite possibly, they may have suffered great tragedy, and maybe even the loss of a son – unless there is a better reason for the moaning in the night. If Jean had been a little older and a little wiser, he would have put this together. He leaves it to us to surmise.
Stereotypes aside, Yoshoto is also important to the story because he is an excellent artist. As Jean is quick to point out, this doesn't necessarily mean he can teach art.