Pastor Otto Falk
Otto Falk is a pastor, and the last person known to have seen Harriet on the day she disappears. When Blomkvist visits him, he's in the throes of Alzheimer's. When Blomkvist asks him about Harriet, he says, "[Harriet's] looking for the forbidden truth. She isn't a good Christian" (22.59).
Apparently, Harriet was identifying women her father murdered in parodies of Biblical passages from the book of Leviticus, concerning burn offerings and sacrifice. We know that Gottfried tells Harriet about the murders the night he dies. Her date book suggests that she was trying to identify his victims after his death.
If she told this to Otto, or if he figured it out, this would explain his cryptic statement to Blomkvist. Harriet isn't interested in the Bible for spiritual satisfaction, but to find the truth of her father's secret life. If Falk does know the secret, this might explain why "his parsonage was burnt down in the late seventies" (21.116).
Martin built his house on Hedeby in the late 1970s. If he knew that Falk knew about his father, he might have burned down the church house to try to scare him into keeping quiet.