The character description of Jonathan says that he has a face "botched" by plastic surgery so that he looks like the screen villain Boris Karloff. Jonathan shows tension and upset about his face when Aunt Martha mentions that it is frightening. On several other occasions when people comment on his resemblance to Boris Karloff, the stage directions say that Jonathan "stiffens." He says to Einstein, who has done the three plastic surgeries on him,
You see, Doctor—what you've done to me. Even my own family [finds me hideous].
When Klein begins to say he looks like Boris Karloff, the stage directions say Jonathan
at the mention of Boris Karloff, leaps at Klein’s throat.
Jonathan is angry about his appearance. Jonathan's resemblance to screen villain Boris Karloff does, however, fit his personality. He is a psychopath, a person whose odd behavior and bad temper would be frightening and unsettling even without the face to match. He is also a murderer who has escaped recently from an institution for the criminally insane. He and Dr. Einstein plan to bury a man named Mr. Spenazo, who Jonathan killed, in Jonathan's aunts' basement, where Teddy, thinking he is digging the Panama Canal, has actually dug a grave. However, the aunts also have a dead body earmarked for the same site.
Even though this is a zany, if dark, comedy, Jonathan still stands out as a creepy character.