When it comes to Korean webtoons, Hellper is a bit of an oddball. Although in Korea it's about as popular as Tower of God (a poll on Naver even put Hellper above ToG), it doesn't get much attention outside of Korea. This is painfully clear in the meager 7 ratings this manhwa has at this time, while ToG has about 2000. So now I find myself here, trying to put in words why Hellper is good and why you should read it.
The story centers around Jang Gwangnam, leader of Killberos. Killberos is a group made up of gangsters, who were gathered by Gwangnam to give them a chance to live relatively normally in society. Killberos was also the reason why there was so little crime in their city: Gwangnam organised them to protect.
... and then, in chapter 1, Gwangnam dies and he becomes a naked ghost with a black ticket - which means that, if a reaper finds him, he'll be sent to Hell. But Sese, the first friendly ghost he meets, tells him that there's still a chance for him: when one collects 100 black tickets, they can be traded for a white ticket, meaning Heaven or Reincarnation. Of course, this means Gwangnam will have to go look for other ghosts, to fight them. Sese also points out that there's something odd about his death, introducing a nice touch of mystery.
The comedy is on point (Gwangnam is one hilarious character), and the build-up of suspense is handled well too. Fights are exciting and unpredictable, there's no "power of friendship", but also no "absolute power".
Something I also like about this series, is how the world of the living isn't forgotten. The consequences of Gwangnam's death for Killberos are also important. This in combination with his actions in the afterlife help to establish the "character" that is Gwangnam.
Speaking of characters: about all of them have some degree of interesting-ness. This is especially true for Sese, Gwangnam's "sidekick" in the afterlife. From all the series I've read so far, Sese has to be one of the most unique characters.
Aside from those two, the other characters are handled well too. A nice touch was the added backstory for a teddybear, and for Gwangnam's granny.
And then there's the last thing I need to address, but certainly not the least: the artwork. If you have taken a look at the character section and the cover pic, you should have noticed it already: the style is unique. Mainly black and white, with often only 1 basic colour added to every person. It took me a couple of chapters to adapt, but once I got used to it could only say "wow, this is great". The art works very well in both the "static" scenes and the fight scenes.
Overall: are you a fan of good stories, with a bunch of cool/interesting characters and a unique artstyle? Then you should definitely give Hellper a try.
P.S.: This manhwa is being translated at webtoons.com, where there are 84 chapters out right now. There are some things that are mistranslations (eg "Black bird" while it's supposed to say "black king"), so be sure to check the comments (most of the time someone will point them out there)