If the player character discovers the secret of the town, by going into either the Wilsons' shed or the Smiths' basement and subsequently kills the residents, Old Man Harris will become the guardian of the children and speak more openly of his history. He is the one who tells the Lone Wanderer about the cannibalism, but the others tell them that he went mad ever since his wife died.
He is the father of both Linda Smith and Martha Wilson; the player character can learn this by passing some speech checks.
If Jenny's and Junior's parents are killed, they will come to live with Old Man Harris. If Old Man Harris is killed after this, the children do not seem to care or notice, even if he is murdered right in front of them.
Killing Old Man Harris will not incur a penalty to Karma.
Even if the rest of Andale's residents are killed, Old Man Harris will continue to say things like "Don't be fooled by them!" during dialogue.
"No! Well... yes. But it's all a trick. Don't you know anything? Did you just crawl out of a vault or something? Just about everyone in the wasteland knows to avoid Andale, and they're right to do it. People wander in here, and they don't wander back out." – (When wearing a Vault jumpsuit)