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 feral 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 the feral ghouls.
He is not immediately hostile, but if the Sole Survivor tells him they will not send over the settlers and that they will be taking over his land, 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, one can also accept his offer and kill him immediately afterward, earning the player character 615 caps and allowing the settlers to live.
However, if the player character accepts Malcolm's offer and chooses to kill him straight afterward, the citizens of Far Harbor will say that the settlers they sent there have gone missing (as a 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.
If the Sole Survivor has any rank of the Cannibal perk, Malcolm will note this when he is asked if he is a cannibal.