Arne was originally a route but was scrapped/reused (considering Torulf and Arne are the only characters with sprites for harnesses, and the fact that many of the character sprites are still in the game files)
Arne is actually meant to be a future route, but I highly doubt that, because that doesn't make any sense
One other guess I got is that maybe Arne was having some sort of affair with Devon (considering they are colleagues, I guess and maybe of age?) but was switched later on.