The Woman in Black
Mr. Bentley is Arthur's employer, the one who sends him off on the fateful trip to Crythin Gifford in the first place. Though Arthur doesn't really blame Mr. Bentley for what happened (how could he have known?), he's still the catalyst for the events that follow:
He had always blamed himself, at least in part, for what had happened to me—it had, after all, been he who had sent me on that first journey up to Crythin Gifford, and Eel Marsh House, and to the funeral of Mrs. Drablow. (1.17)
Because of a mere command from his boss, Arthur's life spirals out of control and into the depths of grief and despair. Wow. Makes you want to call in sick, huh? But it also makes us wonder: how much did Arthur tell him? And how much did Mr. Bentley believe?