Griff the Scavenger is a NPC that is trapped during the High Overseer Campbell mission. Once you get past the first Wall of Light, there is an alley to your right where two members of the Bottle Street Gang are keeping someone hostage inside of a building, which happens to be Griff.
To take them out nonlethally, once the two enemies are done with their conversation with Griff, one of the guys will walk away, which is your perfect chance to take him down. Also, if you don't make it in time, the two gang members will be in their original spots, but it's possible to choke out the one without alerting the other.
Once both guys are down, break open the wood holding Griff inside and talk to him. He'll thank you by offering to sell some items to you. Most of them are just generic items like elixirs and ammunition, but he does sell two Blueprints. Griff is only available in this mission and the next one, House of Pleasure. Note that one of the Blueprints he sells can be acquired later on in The Royal Physician.
