Created by Steve Ditko, the Creeper first appeared in "Showcase" #73 (April, 1968). He went on to star in his own short-lived magazine, suffered a couple of retcons and revisions, and has done some guest appearances, never quite achieving wide popularity.The original story tells about Jack Ryder, a Gotham City talk host who tries to save a scientist named Dr. Yatz from mobsters. An attempt to sneak in to the boss' mansion in probably one of the weirdest outfits ever, where Dr. Yatz was held during a masquerade, only got Ryder wounded and saved by Dr. Yatz who implanted him with a device that could make his costume disappear and with a serum that gave him super strength and agility, as well as the power to heal almost instantly. It's also mentioned that his laughter makes people nervous or is even physically painful. Ryder could activate the suit (and apparently the superpowers, too, since he mentions not being so fast and strong in his normal form) whenever he wanted with a button-like object.Depending on the series, Creeper either acts like Ryder or is happily insane. Some versions explain the insanity by stating that he had a drug of some sort in his system at the time he had the device inserted, so the narcotic is recreated along with the costume.The Creeper is also remarkable in being pretty much the only character The Joker fears, which is one hell of an accomplishment.No relation to the walking green timebombs from Minecraft, or the hunched-over hulk who constantly shouts his name from Scooby-Doo, or the monster from Jeepers Creepers.
Expy: The Creeper's resemblance to Batman foe The Joker (both have green hair, clownlike faces and maniacal laughter) first pointed out in an issue of the latter's own comic book in the 1970's (where the two fought) may have led to the character's reinterpretation as being insane.
Also, consider that The Creeper is remarkably similar to Ditko's own Loki, held over from Thor/Dr. Strange. They are very similar characters.
In the animated series, The Creeper's design and personality is based on Freakazoid!.
Good Counterpart: The DCAU version is this in respect to the Joker, being transformed by falling into toxic chemicals placed by the villain as well as being hit by his laughing gas bombs. Notably, he acts like a Stalker with a Crush towards Harley Quinn, mirroring her behavior towards the Joker.
In the cartoon, the Creeper finally catches up to Joker and Harley at the end of the episode, ending with Creeper hurling Joker away, making him land at Batman's feet. Joker promptly grabs Batman's cape and screams "HE'S A LUNATIC!!", as if begging to be arrested.
In the DCAU, he appears to eventually have joined the Justice League, although sadly doesn't get any more speaking parts.
Happy Rain: In one story he ponders how he's always enjoyed being in the rain, and as Ryder even doing news reports during blizzards and typhoons.
Impractically Fancy Outfit: Kinda lampshaded; he actually chooses and buys his costume, assisted by a bored-looking shop attendant who looks as though half-naked psychopaths leaping in, trashing the store, asking for dress advice and paying by credit card is perfectly normal (well, this is Gotham...)
Absolutely true: she has fans. It's mostly one guy, but it's also hilarious.
It's also the most Stripperiffic of any male DCAU character. It's just boots, gloves, the boa and a speedo. No, really. It makes a bit more sense in the DCAU than in the comics though, since he seems to have a very high dose of Made of Iron in his powerset.
In the original comics the costume was actually a full suit, the yellow part being spandex or similar (and it was also the last suit available in the shop) and the hair being a wig. It was also recognized to be a costume by some criminals, but since the doctor's device makes the costume stick like glue, trying to pull the wig off with no effect made them realize it to be the "real deal".
However, it was used to establish him before his actual origin episode (like with Harvey Dent, who was in at least three episodes before becoming Two-Face) in order to avoid too much Remember the New Guy.
In Batman: Gotham Adventures #58, the Diniverse Ryder is seen to shapeshift if he hasn't taken his medicine or due to a rush of adrenaline. He made the "change with a rush of adrenalin" thing work for him; once he needed to quickly transform, so he jumped out a skyscraper window!
Jack Ryder spent the good portion of one comic book spontaneously half-transforming into the Creeper without any control. His attempts to make himself scarce in the presence of his co-workers were entertaining.
Nigh Invulnerable: In the Diniverse, the Creeper has a near identical origin as the Joker, yet he has managed to survive having a crate containing a rock statue the size of a car dropped on him, and going through multiple explosions without a single injury. Or hint of Clothing Damage. Also, he's able to send Batman flying with one punch, and break through walls. And easily dances around any attacks. Batman explains that this power is most likely due to a strange chemical reaction of the acid and the Joker's laughing gas. The REAL reason for this though is just Rule of Funny.
Plot Hole: Steve Niles' Retcon explains the change with nanocells, but the fact that Ryder can change to Creeper (who has his own set of "clothes") and back and still have his normal clothes is never explained or even lampshaded.
Terror Hero: In the original comics Creeper used to pull a scare shtick by pretending he was not human, describing the horrible fates of his enemies, laughing and acting weirdly to make his enemies panic (and loving every minute of it). This worked well enough that he actually managed to make one less hard-boiled guy faint.
Too Kinky to Torture: Or insane at least, as is found out when he's tortured to test his pain threshold.