His mother's family left the Kingdom of Hannover in the 19th century as asylum seekers, establishing themselves in Roswinkel in the province of Drenthe.

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.

Themes in his work include death, loneliness, marginal figures, the precarious position of Low Saxon, and the difficult relationship between men and women.

[8] He has frequently parodied the work of other Drèents writers, such as Marga Kool, Roel Reijntjes and Peter van der Velde.

From its improvised beginnings, the magazine went on to become one of the house publications of the Drèents emancipation institute Huus van de Taol.

[11] Shortly before starting Roet, Koster had been an editor of De Pennevogel (appearing between 1974–1979) which, on a similar tack as its predecessor 't Swieniegeltje, had included work in several varieties of Low Saxon from the Netherlands and Germany.

Some members wanted to see him expelled on the basis of his writings, but under the patronage of the society's president Klaas Kleine he was appointed to the board in 1991.

In 1995, Het Drentse Boek published his semi-autobiographical account Zölfportret mit sparzegelties (Self-portrait with Spar savings coupons), vignettes largely of his childhood and student life.

He made the first Drèents translation of a comic, the Spike and Suzy album Het hondeparadies (The dog paradise), published in 1998.

It won the 2011 Dagblad van het Noorden regional language prize in the prose and poetry category.

Also in 2020 appeared Een goed stuk vleis: Drents volkstiedschrift under the pseudonymous editorship of Lodewijk van Heiden and John Harbours (Koster and Jan Harbers).