"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Keir was a Nazairi bandit captain and part of Syanna's gang in 1275.
As Geralt was closing in on learning the truth behind the murders going on in Toussaint, Syanna sent Keir to try and kill the witcher.
If Geralt walks into the ambush: Unfortunately for Keir and his friends, he met his end in Beauclair after attempting to ambush Geralt.