Harsen was a strange, antisocial man who came to the Island and rented a room in the White Lion. Although initially the pub's owners Richard and Frankie Callum suspected him to be in league with a race of invading alien spiders (due to the fact they'd seen him skulking around near where Patricia Wells had said she'd seen a flying saucer land), Harsen eventually revealed himself to be a scientist who was trying to gather proof of the aliens' planned invasion of Earth before it was too late.
When Vernon Stone's attempt to reach the Island's radio station to call for help failed, Harsen went after him and found him dead in his overturned car, burned to death from contact with the radio-materialized spiders. Instead of heading immediately to the radio station himself, Harsen attempted to bait into the headlights of his parked van the spider that had killed Stone, hoping to get a look at the creature. He got more than he bargained for when the alien refused to be tricked and instead attacked him, and the unfortunate scientist met the same fate as the author.