Blue Devil is a superhero featured in material published by DC Comics. He first appeared in a preview story published in Fury of Firestorm #24, cover dated June 1984. That story led directly into Blue Devil #1, also cover dated June 1984.
Daniel Patrick Cassidy is a special effects wizard and stuntman hired to create and play the title character in the movie Blue Devil. To that end Cassidy creates a full-body costume with a hidden powered exoskeleton and built-in special-effects devices. When two of his co-stars accidentally free a demon named Nebiros, Cassidy uses his costume to drive the demon back, but not before being blasted with mystical energy. After the fight, Cassidy finds that the blast had permanently grafted the Blue Devil costume to his body.
Cassidy quickly finds himself embroiled in repeated adventures and conflicts with supervillains, as his newfound status has turned him into a "weirdness magnet." Never stopping to seek a way to remove his costume, Cassidy starts to enjoy his new lifestyle, and becomes more and more comfortable as a superhero, even accepting a place in the Justice League of America. He gains a sidekick of some sort, when Eddie Bloomberg, his former gofer and devoted fanboy, designs a suit similar to his own to adventure as the Kid Devil.
Opposing Neron, Cassidy is able to partly foil the demon's short-terms plans, but he dies in the process. Cassidy's death is short-lived, as he is restored to life as a real devil, no longer a man in an organic suit. Ironically, Cassidy had been a devout Catholic, but now the Blue Devil is despised by the Catholic Church and even unable to set foot on consecrated ground without self-immolating.
Blue Devil joins the Justice League again, until his second death at the hands of Mist, who showered him with holy water. Blue Devil's bones are retrieved by Sebastian Faust, who much later restores him again to life, seeking help to restart the fires of Hell during the Day of Judgment storyline. Blue Devil now has the chance to confront Nebiros, empowered by a magic trident seemingly given by the King of Lies himself; Blue Devil stalls Nebiros, stripping him of his trident and allowing Firestorm to deal the killing blow, while back on Earth the other heroes of the Sentinels of Magic save the world.
Blue Devil then joins the Sentinels of Magic and partners with Faust, but it is an uneasy partnership. As an apparent act of kindness, Faust restores Blue Devil's freedom by restoring to him his last missing bone. Blue Devil sacrifices his life against Hermes Trismegistus, but as a true devil he is restored to life again, tasked to roam the Earth with Lucifer's Trident banishing escaped demons back to Hell.
He spends the missing year trapped in Riverrock, Wyoming along with the rest of Shadowpact. It is a small city hidden by a blood shield by an assembly of evil magical beings, called the Pentacle. There he meets, or rather meets again, Jack of Fire, a red, muscular demon. The entity is hiding a disfigured, bony face under a black bandanna, claiming to having being turned into a demon by the actions of Daniel. He further explains that the very same moment the demonic Neron granted fame and powers to Cassidy as the Blue Devil, the dead parents and siblings of Patrick were dragged from Heaven to Hell.
From then on, Blue Devil's life starts going downhill: even if in his personal time line passed only a few days, the Enchantress was forced to steal a year of life out of every Shadowpact member (and surviving Riverrock towns person) to shatter the bloody barrier. As such, the entire world had long thought Shadowpact lost, even building a memorial statue. Having been considered dead for a year, Blue Devil struggles to rebuild his life.
Blue Devil is forced to confront Kid Devil, now granted meta-human powers by Neron himself. Ultimately, Kid Devil's love for Maria helps break the deal with Neron; if he had kept his loyalty to Blue Devil, his servitude to Neron would be negated.
To add further misery to his condition, Blue Devil is demoted to the rank of 'rhyming demon'. His new status results in the heroic Earth-based angel Zauriel being sent to confront him. His demotion is explained the paradoxical condition of his being both a hero and a demon: being famous and loved by everyone, he makes Hell a glamorous place, spurring weak minds to attain his glamor by selling their souls to hell. Zauriel, on behalf of his good faith, takes Blue Devil's place in the Shadowpact, giving him enough time get out of his infernal deal.
Blue Devil puts his case in the hands of an eager lawyer, more than happy to be able to confront Hell itself in an hall of justice. Blue Devil undertakes the onus of completing 13 labors, as Hercules before him, on behalf of the Catholic Church as a show of good will.
Returning to Hell for the final hearing, he discovers that Jack of Fire was the one selling his sibling's soul first, with Blue Devil merely following unwillingly. His lawyer repeatedly proves that Jack is the only one who can return Cassidy's soul, and the souls of his family, back, since he was the one who sold them in the first place. Blue Devil uses his new lower status as a rhyming demon to force Jack to do so, but in the process he loses both Lucifer's trident and his demon form, reverting back to a mortal.
Jack of Fire is now allied with a seemingly all-powerful Sun God, but plots to betray him in retaliation for his harsh treatment and to lead an army of demons on Earth. Cassidy feels forced to don a new suit of armor, very similar to the one he wore during his Hollywood years, but even more powerful. He sets off to fight his brother, who refuses. Instead, after a brief confrontation, acknowledging that the Sun God will eventually kill him for his betrayal, he commits suicide in front of Cassidy, naming him inheritor of all his powers and demonic status - but without forfeiting his soul. Cassidy rejoins the Shadowpact to defeat the Sun God, and Zauriel ends his temporary membership with the group.
In his current incarnation, Blue Devil still possesses some degree of superhuman strength, capable of going toe-to-toe with beings like Eclipso, and knocking out a power-drunk Enchantress in one punch. He is also remarkably resistant to physical damage, evidenced when Eclipso threw him a distance of approximately 500 feet upwards, he didn't even require time to recover before getting up and heading back into the fight.
Originally, Blue Devil wielded a trident of his own design which included, among other things, rocket engines capable of carrying two people at high speeds. He now carries the Trident of Lucifer, which allows him to find demons on Earth and banish them to Hell. During a battle with Eclipso and the Specter the Trident was thrown into the ocean; it was recovered shortly after by Aquaman who returned it to Cassidy, and much later given to Jack of Fire.
Briefly resuming his human body with a more powerful exosuit, Cassidy is given back his Trident and demonic powers by Jack of Fire, but as a "demon with a soul".