|Wait for it ...|
I've written many times before about the heroes with the toughest origin stories of all -- the ones who had to die before they gain their powers and their mission against crime/evil/injustice/parking violations and so on. For the most part, they fall into two categories*: cops and kiddies, with some outliers like Deadman making a decent stand for assassinated circus performers.
* And "historical medieval-type guys," I know, don't @ me, it's just the two (Gay Ghost and Mr.Justice) and the latter of them was an English prince murdered by Scotsmen, so I'm on their side.
|"Ho-hum, I'm dead, better crash on the couch here..."|
Created and originally crafted by Martin Kildale** and running for an obscenely long time in Blue Bolt Comics (plus a few others), Sgt.Spook literally passes through this mortal veil because he blew himself to hell. Unwisely smoking in the Forensics Lab, Spook (real name: Sgt Spook, evidently) absent-mindedly puts his still-smoking pipe down on top of a stack of volatile chemicals. Kapow! He's dead, the end.
**Kildale was also the creator of y'boy Speed Centaur, the only superhero whose secret identity involved putting on a huge rubber horse head mask and hauling young lovers on romantic tours of the park. That I know of.
Without any sort of explanation, Spook finds that he's become a ghost capable of interacting in a limited fashion with the real world. While he's completely invisible and cannot be heard by the living, he can also either pass through solid matter or become solid enough to interact with the material world, such as to slug a crook or hold onto a crook or hold onto a crook long enough to slug him.
Sgt.Spook picks up a genuinely interesting gimmick later in his run, as he begins to police the actions of other ghosts -- BAD ghosts -- from history who are using their powers to continue to commit crimes in the world of the living. You almost get the feeling that it was his destiny all along, but he jumped the gun on snuffing it and had to wait around, stopping quotidian gangsters and common crooks until the real reason he's become a wispy force of justice came around...