leopold99 said:if i am not mistaken leopold was the name of a german gun used to destroy warsaw or parts of france during world war 2.
Leopold's claim to fame was being half the battery of guns known as "Anzio Annie" to American troops being bombarded by it and it's twin - named Robert - as they attempted to establish a beach-head near Anzio, Italy during WW2

Leopold is currently displayed at the Aberdeen Ordinance Museum in Maryland
As for me...
Killjoy is a combination of a play on words centering on a bit of graffiti painted by US troops on various locations across the globe - "Kilroy was here", and the name of an obscure character from the (I believe) now defunct Charlton comics publication "E-Man".
Killjoy seemed to have no super powers save the ability to nullify the effects of whatever villains attempted to do to him, at which point they might be inclined to utter the phrase:
"You're such a Killjoy !"