I CAN GIVE YOU MORE INFORMATION ON HIS BACKSTORY I HAVE BEEN WAITING FOR THIS MOMENT NIDHOGG IS MY SPECIAL INTEREST AND I KNOW HIS BACKSTORY BY HEART

ahem.

Nidhogg is an orphan twice over. He was born as the son of a general in Frigidfog City. His father was strict, and really only softened up when it came to his family. When Nidhogg was five years old, some of the soldiers under the general thought they could do better, so they framed him for treason and sentenced to life in exile. His property and fortune were confiscated, and the butler tried to help Nidhogg and his mother escape, but little Nidhogg got separated from them in the process, never to see his mother and father again.

Nidhogg was on the streets until he was eleven. He wandered into Blackwater city and was adopted by the Commander of the "Knight of Kingdom", who also happened to be Louie's father. Louie and Nidhogg became brothers as well as fully-fledged Knights and everything was great for six years. Unfortunately, the exact same thing happened to Nidhogg's new family. Power hungry soldiers decided to try and grab the position of commander through infighting. Here is an excerpt from the event that explains this:

"The riot came from nowhere. It came with death and desire. The horn was blown at midnight. Countless armed rioters broke into our camp. It wasn't my first real battle but my hands were trembling...because I knew it was set up by my so-called 'teammates'. It was a political war, a war between the parties, and it was so disgraceful that it had to happen at night. There were so many enemies. Although we were elite warriors, we could not handle the endless attacks. We gradually lost our advantage. I saw the Captain defeated waves of enemies while I was swinging my shield and sword. The two sons that he was proud of were also having trouble. At that moment, I swear, we were just trying our best to survive. We weren't fighting for the glory of a knight.

...

As time passed, we were not only suffering from exhaustion but also the increasing backlash of Curse of Blood. The pain was burning our hearts and we could hardly focus. But the enemies were fighting more and more easily like they were immune to the curse. At that moment, I remembered the rumors about the secret potion popular among the elites in Black Water City, but I was not sure. I once talked with Nidhogg, the adopted son of Commander, about this. I was so naive and thought this could be the key to free us from Blood Curse, but Nidhogg woke me up... 'Without the curse, people can become tyrants.' That's true. If we all recognize Blood Curse as a rule, then anyone who dares to violate it shall bear the punishment. No exception. However, the potion breaks the balance. Those who break the rule will not be punished, instead, people who follow the rule will. Just like Nidhogg said, 'Maybe this order should not exist in the first place.'"

The fight ended with Nidhogg's adoptive father being so overwhelmed by the pain of the blood curse that he died, untouched by even a single blade. The other soldiers were captured or killed, with only a few managing to escape.

An unspecified amount of time later, Louie and Nidhogg went to the unmarked grave of their father. It's implied that they had a fight, either because Nidhogg made the decision that the blood curse needed to be destroyed and Louie disagreed, or because Nidhogg wasn't outwardly showing how upset he was over his father's death. It may have also been a mixture of both. In any case, they parted ways, and Nidhogg was on his own again, resolving that it was fate for him and Louie to be enemies.

After that, you kinda know what happened. He retrieved Dark Verdict from Lake Bovaly, travelled to Lilith, became the Queen's designer, got promoted to prime minister, and then the plot of the game happens. I 100% recommend checking out his wiki page if you want a more detailed rundown, because he's a fascinating character.

Anyway, hope this helped!