Koster's poems and sketches, which have by and large appeared in Roet magazine, were published under a plethora of playful pen names, including the prominent one Meine Roswinkel.
With fellow Drèents writer Anne Doornbos, Koster founded the 'Nei Drèents Geneutschap' in 2005, a society which gives awards to people and organizations for doing the Drèents language a good turn.