Mr. Excitement was the Frank Einstein, Madman's, favorite comic book superhero. This was all Madman could remember of his former life besides faint details of his death. Soon after his reanimation, Madman became self-conscious and depressed by his Frakenstein-esque appearance. Joe gave him the idea to adopt a costume and mask (whose interiors were treated with a special agent of Dr. Flem's, designed to heal Madman's massive scarring), which he styled after Mr. Excitement (most apparent by the exclamation point logo on his chest).
In Madman Atomics Comics #3, Mr. Excitement appears to Madman to help lead him to reconnect his mind with his body. By the end of their travels, Madman realizes it is actually Astroman, who has had his brain wired to Madman's in an attempt to save him.