Malcolm is a lone trapper living in Echo Lake Lumber. He went there to escape the Fog, but eventually accepted it and let it turn him insane. He makes offerings to the Fog. He is the 'owner' of the area, but the ghouls drove him out. He comes out of hiding when they are killed.
He will spawn and start talking to the player character once Echo Lake Lumber has been cleared of all ghouls.
He is not immediately hostile, but if the Sole Survivor tells him they will not send over the settlers he will become hostile.
During the quest Turn Back the Fog, he will say he will ambush the settlers when they arrive and eat them. This prompts the player character to accept the caps and let the settlers die, or tell him he can keep his land, or refuse and kill him. However, you can also accept his offer and kill him immediately afterwards, earning the player character 615 caps and allowing the settlers to live.
However, if you accept Malcolm's offer and choose to kill him straight afterwards, the citizens of Far Harbor will say that the settlers they sent there have gone missing (as flag is set when picking a choice at the end of the dialogue). This will have a negative effect during a possible confrontation in Far Harbor for one of the main quests.