They are real people (though I don't know whether they are likenesses)
The monument commemorates four people - members of the resistance - shot by the occupying Germans during the first world war. It was dismantled by the Germans when they occupied Lille again in WW2, but pieces of the monument were preserved by the locals and it was rebuilt after the war.
The momument's title means "Lille, to her people who were shot". I don't know whether the four specified were the only ones shot during WW1, but the fifthe figure (on the right, on the ground) suggests otherwise; presumably much the same thing would have happened during WW2, so the monument would commemorate citizens of Lille killed during that war, too.
More about it here, in French:
http://www.cheminsdememoire.gouv.fr/page/affic heLieu.php?idLang=fr&idLieu=1347