The 18-year-old, born in Malaga to parents originally from Barrhead, is progressing steadily through the ranks at the Bernabeu and could yet be could yet be joined in the competition by Ryan Gauld, also 18, should the former Dundee United player be named in Sporting Lisbon's squad.
The Youth League, about to begin the second year of its two-year trial after replacing the NextGen Series, mirrors top-team matches and encourages youth squads to rub shoulders and share flights with their clubs' senior stars.
While the path to the first team is obviously particularly cluttered at clubs such as Madrid, who recruited James Rodriguez and Toni Kroos this summer, it isn't totally impassable. Only last week, Munir el Haddadi, an 18-year-old of Moroccan descent whose audacious strike from halfway illuminated last year's Uefa Youth League final, made a fine goalscoring La Liga debut Barcelona.
Unfortunately for his Scottish audience, while Gauld is on under-21 duty in Luxembourg and Slovenia, Harper won't be at Cappielow on Tuesday when the national under-19 side take when the Czech Republic in a friendly.
The match clashes with his duties for Madrid's Juvenil A side at a prestigious youth tournament at Ourense, in Spain.
The prolific scorer for Madrid's youth teams, who can play either in the main striker's role or just behind it, still has three years remaining on his contract and seems certain to be a central figure for Scotland in years to come.
Harper, whose younger brother Mac is also highly-rated, experienced his first taste of injury last season, but will hopefully be able to put that behind him this term.
His older brother Ryan, 27, once rated highly enough to hold down a striking role under Pepe Mel at Real Betis, has retired from the game this summer after falling victim to his third cruciate ligament injury of his career. He is now studying for a degree in the business of football management in preparation for continuing a career in the sport.