He makes his first appearance in Biggles Works It Out:
“[Marcel] was so obviously alive, and so glad to be alive. His vitality revealed itself in every eloquent gesture that he made—and he made many. Enthusiasm and good humor sparkled in his dark eyes when he spoke; and there were moments when words fairly rattled off his lips in his anxiety to give expression to his thoughts…every passing emotion was reflected instantly on his face.”
--Biggles Works It Out
He introduces himself as “the French Air
Police”, and to Biggles’ amused query, “Just you?” replies happily, “Oui, Monsieur. Just me.”
And as for his physical appearance:
“…about twenty-five, slim, with fine, clean-cut features and a miniature black mustache decorating his upper lip. His general appearance was neat, without being foppish…in a word he was just as typically French as Biggles was British.”
--Biggles Works It Out
Marcel was mainly added to the air police books to create the idea of an international police force; when crooks cross over to France it is usually Marcel Biggles calls for help, and sometimes when French crooks stray into British territory Marcel drops in on his friend "Beegles" (which is what Marie called him too. Hmmmm. *Long Heavy Silence*) and asks for his help.
Sadly it is only in Works It Out that Marcel seems refreshing and interesting. As time goes on he grows increasingly normal, until in the end he is just a name mentioned in passing whenever something happens in France.