Demon Slayer Backstory's (Cannon)

Rui

Word Count : 517

Rui was born a very sickly kid. His body was very frail and the slightest cold had the ability to kill him. Because of this, his parents were very protective and almost never let Rui out of the house. One day however Rui got annoyed with not being able to go outside, left the warmth of his house, and went outside into the winter chill of the day. While he was outside he started watching a group of other kids playing with a ball, sad he couldn't join in. However, because his body was so frail he obtained a fatal cold that caused him to be bedridden for days. After a few days of Rui laying in bed a man walked into the room and started asking Rui if he wanted a chance to live a healthy life. Rui immediately responded that he wanted to live a normal life, this man was Muzan Kibustuji, and because of Rui's response, Muzan gave Rui some of his blood turning him into a demon. 

Now a demon Rui was able to move around freely and without restraint, but because he was now a demon he need to eat humans for substance. Because of this Rui ended up killing and eating a man that had walked into the family's home. When Rui's parents came home and saw the bloody mess that their son had created they were instantly discussed. Later that night, Rui woke up to see his father standing above him holding a knife whispering something, alongside his mother sobbing her eyes out. When he saw this he mortally wounded both of them and sat outside, reflecting on his and their actions. He recalls hearing a story about a man who drowned to save his son, and how he was moved by the act of parental love. He reasoned that because his parents tried to kill him, they must have been imposters, refusing to believe they could even think to consider killing him. However, as his mother lay dying, Rui overheard her apologizing for not being able to give him a healthy body, which then caused him to remember his father's last words, promising that both he and his wife would follow Rui to the afterlife by killing themselves. He came to realize that his parents did indeed love him and simply tried to stop him from fully becoming a monster, throwing Rui further into emotional turmoil. Muzan appeared shortly afterwards and told the young demon that it was his parents' fault for refusing to accept his new form. Unable to bear the weight of his actions, Rui pushed Muzan's words as the truth and believed them.

As a demon, Rui lived primarily in the forests of Mount Natagumo. Over time, he grew to become a very powerful demon, armed with both a high concentration of Muzan's blood and an extremely lethal Blood Demon Art; durable, razor-sharp threads that he could unleash at will and with great proficiency. He attained the rank of Lower Five among the Twelve Kizuki but did not care for rising up in rank.