Koster is the Dutch word for sexton or verger, derived from the Latin custos (the equivalent German word is "Küster").
It became one of the wealthiest monasteries of Hainault and variously founded, or was given the supervision of, several daughter houses: the abbeys of Fontenelle at Valenciennes (1212), Nieuwenbosch near Ghent (1215), Épinlieu at Mons (1216), Beaupré near Mechelen (1221), Le Refuge at Ath (1224), Le Verger at Cambrai (1225) and Baudeloo at Saint-Nicolas (1225).
He is a member of Pommeraie family, from Britanny and serves Laval Family, and possessed land of Verger Castle of Montigné and d'Entrammes (Mayenne).