Who is Stuart Rene ("Stu") LaJoie from The Moon Is a Harsh Mistress and what is their importance?

A self-styled "Poet, Traveler, Soldier of Fortune," the aristocratic French royalist LaJoie needs protagonist and narrator, Manuel ("Mannie") Garcia O'Kelly-Davis' help when he visits the moon as a tourist and commits a mortal faux pas. The situation is typical: a freely-wandering tourist reacts in "earthworm" fashion by trying to kiss a woman who flirts with him; she screams in typical Luna fashion and the stilyagi come to her rescue. Davis sees no offense but fines LaJoie $50 for ignorance of Lunar ways. LaJoie, who reminds Davis of Prof, asks about "tanstaafl."