I read this manga sometime around 2005-2007 on one of the many "free online manga" websites, and at the time this was translated into English from Japanese through a fan translation group.
In this story, people were able to enter a virtual reality world in the same style of Sword Art Online or .hack. The gimmick here was that if you died in the game you could go back to your body and keep your progress, but if you logged out while dead (you may have needed to stay logged out for 10 minutes or so), then your character was reset back to level one or deleted. The main character made a living by being an in-game hit man. In the first chapter, he was hired by a family because their son was spending too much time inside the game and would not listen to their pleas to spend time in the real world. The main character would go into the game and player kill the target over and over and over each time they re-spawned until they eventually give up and logged out while dead. The parents then thank the main character, as their son has given up the game out of frustration.
The second plot point of the series was that players could customize the look of their characters or upload a design file. More rare or unique/designer looks could be worth some money and there were groups of people in-game that would try to immobilize a player so they could make a hi-definition scan of their character to sell. The main character saves a female player from one of these groups, and the main character decides that he is done with being a hit man and joins up with the girl and her party to explore the rest of the world.
The main character has a special ability where the lower his health becomes, the higher his stats get. This is unique because he is a very high level character, and the special ability he has is disabled if you ever die. I remember in the second or third volume, a group of bandits learns about this and attacks the main group, swarming the main character to try and kill him and remove his power. This culminates with
the main character hitting a single health point, showing off the reason why he has lasted so long is because he explodes and kills everyone within a very large radius, leaving only himself alive.
The name of this ability in the fan translation was called "guts", which makes it hard to look up with the sheer number of MMO-themed manga and a character in Berserk having the name Guts (who also is a mercenary). The main character's hit man character wore a very large suit of armor and was a well-known rumor among the player base, with some people not believing he existed but no one wanted to meet him.
Unfortunately I do not recall anything about the rest of the party except for the girl's character had a desirable look that some (one?) groups wanted to steal and sell.