Jonnie was a dog resembling a Husky or Hokkaido dog, it was small brown haired and wore a leather collar.
Jonnie was the pet dog of a man who called himself the Demon Beast hunter after encountering a feline like Devil Beast, killing it.
Jonnie's nose adapts to being able to hunt down Devilbeasts and Devilmen, due to this several Devilmen are killed some of which worked for the H.A. Lady is sent out to deal with the killer and combats the Demon Beast Hunter who slowly turns into a Devilman himself over the course of the fight, however after beating Lady he notices Jonnie growling at him he asks why and looks down into a shard of broken glass, realizing this he kills himself. What happened to Jonnie afterwards is unknown.