Coën was a witcher from Poviss who spent his first winter at Kaer Morhen when Ciri was in the keep. He trained Ciri in sword combat. His features are described as unusual for a witcher, as he retains scars from early childhood diseases, like chicken pox, which he covers with a beard, while most witchers were immunized before contracting such illnesses. This would suggest that he either contracted the disease unusually early in life, or that his witcher training began later than most.
Coën died in the Battle of Brenna (as Ciri foresaw), stabbed with a twin scythe. As a matter of fact, Shani recalls that he died on her operating table. It seems that the Gardener was also present at the time of his death. He joined the war as a volunteer, he was stabbed through the heart in battle and had his aorta split open, but continued to live until the squadron brought him to Milo Vanderbeck, a halfling and a field surgeon who was better known by his nickname, "Rusty."
In the game, the gardener at St. Lebioda's Hospital relates a story about a silver sword belonging to another witcher named Coën. Shani also fills in some details about Coën when Geralt asks her about the sword.