He grew up on an island that had no authority or parents (kinda like an island version of Meteor City without any adult supervision, if there is any), the island is pretty much in a constant state of anarchy. It was an overcrowded island with lots of teenagers and young adults. It was also extremely violent and ppl died there everyday, but the weapons used to commit these murders were mostly swords instead of guns as there were pretty much no guns on the island and the ppl of the island have a deep culture and history of swordsmanship so there is an abundance of these swords. They are made and sold in the underworld of the country so the gangs and cartels of this country use swords instead of guns. The island is considered a humanitarian crisis as there is no government and any attempt to make one usually results in the death of the person trying to bring a change. You could say that it is a country ruled by gangs and organized crime, cartels, and chaos even though there is no official government. The top gangs of the country are as organized, powerful, and brutal as cartels. There are also unorganized gangs where ppl just run around killing ppl for the smallest of things (like an argument, stepping on somebodies shoes, bumping into someone without saying excuse me etc.) and they are offshoots of the organized gangs and cartels.
Now this is where the user comes in. We'll say his name is "Law". Law was exceptionally gifted as a child and he studied medicine with his family who were one of the few doctors on the island. He had a photographic memory and was smart enough to be considered a full fledged doctor/surgeon at the age of 12. A week Law passed the exam to be considered a certified doctor/surgeon his family was killed by an upcoming cartel gang over a drug dispute. He had a mom and dad, two older brothers, and a baby sister (she was about 2-3 years old), they were all killed in front of him. However since Law was an incredibly smart child he was allowed to work as a doctor for the same cartel that killed his family. For the next 15 years he would be traumatized by what happened to his family. While he was working for the cartel that killed his family he had to do all sorts of things. He had to do things like treating stab wounds, amputating people's limbs after they had been nearly cut thru, trying to reattach limbs that had been cut off, disposing of dead bodies of enemies and allies, removing organs from people's dead bodies (sometimes ppl were even still alive and he had to remove their organs). He had lost a lot of patients and there was death and chaos all around him. While growing up in this chaotic environment he took a vow to never kill anybody despite what anybody tells him and to treat anybody who needed the treatment of a doctor.
This vow of his almost led to him being beaten to death on multiple occasions for not killing when told and treating anybody who asked for his help. He pushed this ideal to its limits and became the best doctors/surgeons in the county (and one of the best in the world). He was able to treat anyone's wounds, he could amputate calmly and precisely, he could perform complicated and complex surgical procedures by himself and with great efficiency, he could even remove organs cleanly and sometimes without killing the patient, and he was also able to dispose of dead bodies without even blinking or showing any emotion (even though he still hated doing so and hated death in general). When he turned 16 it came time for him to buy a high-grade katana as was tradition for any gang member in his country (younger ppl are in gangs and use swords but they usually use very low grade katana or other very low-grade swords, they call them throwaways because they are low quality and are usually thrown away after they are used to kill). He was very opposed to this at first but after he was almost killed himself by an enemy he learned that he would need a way to defend himself. Once he bought his sword and trained in kenjutsu he developed a fighting style that was designed to not kill his opponent but to slash and maim them. When he had to fight someone (usually to defend himself as he wasn't sent on missions like rest of gang members) he gained a reputation for being a great fighter. He became known as the "Dark Doctor" or the "Surgeon of Death". The latter was ironic because he never killed anybody. Instead when he fought his aim was to maim or amputate an opponents limbs to disable them and to keep them from fighting anymore (this fighting style reflected his skills as a surgeon). He could also treat the person he cuts if he so chooses and he usually did because not a lot of people wanted to fight him after he gained his reputation.
When he turned 24 the cartel he was apart of was falling apart and the leader was about to be killed by a rival leader. The rival leader offered Law an opportunity to fight with them and kill the leader who was the same one who ordered the death of his family. Law declined the offer as it went against his beliefs and he decided to leave the island to take the hunter exam and become a treasure hunter. His goal is to find a legendary treasure which is rumored to be on an island in the seas of the outside world and use the money and whatever else is apart of the treasure to economically uplift his country, eliminate the cartels who run it, and establish a powerful and democratic government dedicated to the prosper of its people and all people (after they get their act together of course). The treasure is that of a legendary and ancient pirate crew who sailed the seas around the outside world and the dark continent who successfully returned from their journey telling of the things and riches they found but were killed off by the government of their country to protect a certain secret. Before the captain died however he gave one last speech as to where his treasure and the map to it are. This started an age of exploration all around the world to find the legendary treasure and go to the outside world but it ended in disaster and a huge catastrophe occurred in the human world. Ever since then it has been taboo to talk about the treasure and the outside world (eventually illegal to go there or even talk about it in most countries).
At the age of 25 Law easily passed the hunter exam. Later on he met one of the top 5 users (what incredible luck he has) and they taught him nen and he developed the following hatsu in a period of about 5-6 years. At the end of his training he is considered by some to be the 6th strongest nen user in the world (this is a debated topic in the HxH world and Hunter association, mostly by ppl who haven’t felt or seen his power) but he is extremely powerful regardless. He also learns through from his teacher that he has subconsciously been using nen ever since he was born which is why he is so powerful.
After reading my post u definitely won't lol.
I think you mean to say like indoor fish times 1 million 
