He is described by Hannay as being "five foot ten, very thin and active, and as strong as a buffalo [with] pale blue eyes, a face as gentle as a girl's, and a soft sleepy voice."
Pienaar appears in the following "Richard Hannay" novels: Greenmantle , The Island of Sheep, and Mr Standfast.
Near the end of the novel Mr Standfast, Pienaar - who joined the Royal Flying Corps in 1916 and become something of an "ace" and a hero to the younger pilots although modest and self-effacing as a man, is killed in an aerial collision during an heroic battle with the German air ace "Lensch" (also portrayed by Buchan as a chivalrous and honourable enemy, for Pienaar met him while in brief captivity previous to this story, and Lensch saw to his well-being).
The event is witnessed by General Richard Hannay on the ground, in his trench lines, a short distance away.
[1] The plot of The Island of Sheep arises from an incident in South Africa involving Hannay and Pienaar.