This is a rough idea for Yami's backstory that i thought up on the spot randomly, this will effect his personality of course and will probably explain some latter actions. This is extremely rough and when i do the flashbacks and stuff some parts may be changed .

Yamis backstory rough idea 

Amidst the sea of red and flames stood a lone survivor slathered  with the blood of many on his entire body, with only slight burns and injuries was a young child one of the few that survived the disaster. Having lost his friends, classmates and even his last remaining member of his family in front of his eyes that all died one by one through various means, through the terrible disaster, the sea of flames or even at humans hands that desperately tried to survive or became instead which caused them do drastic measures. Having only survived due to his only remaining relative alive at the cost of her life, his older sister having resorted to drastic measures within the disaster to try and survive at all costs along with her younger brother having even resorted to killing. 

She was the pillar of mental support for Yami at the time who was called a cursed child that brings death, by his surroundings especially his father having killed his mother when he was conceived, and later on people who he knew started to die mysteriously his neighbours, his friends and etc..  Eventually with time he was treated as the cursed demon child within his home town, causing him to lose hope within life and thinking about suicide and death, as he suffered from bullying from his surroundings including the adults, neglect from his father who frequently beats the crap out of him and when he stops the beating he just plain outright neglects Yami. The only one that stuck by his side and gave him support was his older sister she never treated him like a cursed child but rather a blessed one. But because of her actions she was thought to be dealing with the devil and so was treated harshly as well but she never gave up and always smiled. Until the day when his father came home dead drunk drinking his sorrows away on his dead wifes anniversary he couldnt handle the pain anymore and died. His cause of death was thought to be suicide as he was found hanged but the truth was different he died from the hands of ….. Although people thought that he couldnt handle it anymore the pain that he has faced but neverless they still blamed Yami but was they right?. 

You could be reading stolen content. Head to Royal Road for the genuine story.

Later during the year Yami and his older sister was put in a government funded orphanage for special cases, in which they were far away from their problems of their hometown. The orphanage was an institution and so they help treat the mental problems and physical problems of the children or teenagers even some adults. As the treatment went under way Yami was seen to show signs of improvement from his lifeless state to ones of normal children until the disaster happened not long afterwards. 

Being one of the few survivors left from the disaster and at the cost of his mental support his older sister he had a mental breakdown. He started to see the world as it is the real world the cruel reality of society and the fate that is bestowed upon the weak. He started to become more and more deranged in which he no longer cared about the title Cursed child/demon child instead he started to embrace the darkness within, he became a person obsessed with survival as his sister told him to live on and find a happy life, but most of all he became obsessed with vengeance and truth. 

He latter was adopted by an underground organisation that seeked out viable candidates to become an assassin or their pawn. They were lurking for the survivors of the disaster as the incident would have already done part of the work for them, to change their personality wouldnt require much effort due to the disaster. Since they were already broken or was suited already. As Yami was taught by the organisation under harsh measure of survival of the fitness, and one shot one kill meaning you have only one chance failure equals death. 

and i got too lazy to continue thsi rough idea so it will become more brief, 

years later Yami had defected from the organsitation due to an event. During this time period Yami met Hikari. 

author notes

i havent been writing lately as i havent been bothered to actually write as i have to lay out the ground work before i can do the stuff i really want to do but that makes me lose enthusiasm a bit so.... Anyway i would probably start writing soon.