Jack Griffin is completely invisible and wears clothing to be visible. He's a scientist who may have lost his mind, though it could be because of all he has been through since being invisible.
He's fairly serious, and untrusting or suspicious of others due to betrayal in the past. He's wary of anything offered to him, and isn't afraid to threaten anyone with killing them if they wrong him.
He's a man from the 1930s and acts like it, and speaks like it, minus the views of women from that time.