This story isn't fully canon. This is what I have come up with thanks to canon information and fanart. Be prepared. You have been warned. ( Sorry if any of this is weird, it was all I could come up with. )
Kain was born on July 28th 1989 in Midgard's Hospital. On that very same day, his brother Niak was born. They were twins, but not fully identical. Their mother had blonde hair and purple eyes whilst their father had brownish hair and red eyes. Kain got his eye colour from his father and his hair colour from his mother. Niak got his eye colour from his mother but his hair colour was black. It was mainly down to when their father dyed his hair black as a disguise for a party the same day their mother got pregnant. When they were born, their parents had name tags. Alice and Matt. Niak was the first to notice this because he was born first, therefore being older than Kain. Half an hour to be exact.
For the first two years of the twins' life, they cared for each other, played with each other, became friends with future Hunters and even met someone they didn't want to meet again. Kain was outside in the garden by himself with his small puffy coat on. His brother was inside with the flu and couldn't come out. Just as Kain was about to go back in, a grown man grabbed his shoulder and turned him around. The man was Vincent. He had darker hair than present day and yellow eyes. Vincent asked Kain for his full name. Of course, being the good child he was, Kain said his full name. Vincent's last words were “Steele, what a creative surname...” before he walked away. That was the day Niak snapped. He had kept his cool in front of his brother ever since they were born but he had enough.
Over the next few years, Niak started picking on Kain. He told him that the death of their dad was his fault. Kain knew it wasn't his own fault. It was a car crash a week after their birth that killed Matt. Kain's sadness overcame the anger and he cried every time his brother was mean to him. Every year, Niak's abuse towards Kain became worse. From bullying in school to fist fights at home. Alice tried to stop Niak's behaviour, but he kept on doing it.
When both boys hit adulthood, Kain and Niak left home to find jobs. Kain became a famous bodyguard whilst Niak became part of a killing gang. Niak lost part of his sanity every day and one day, he was fully insane and hatched a plan that would make him the most powerful member of his gang and his family. He ran off towards Midgard's snowfields to sabotage Kain's final assignment.
Kain was killing monsters left and right whilst his mother, the woman he was protecting, was holding onto him. All of a sudden, a flash of light bounced off a metal plate on the ground and went directly into Kain's eyes, blinding him temporarily. Niak took his chance. He launched out of the bushes and stabbed Alice three times. One in the pelvis, one in the stomach, one in the chest. She collapsed and died on the spot. Once Kain had his eyesight back, he turned around to notice his mother, dead and in a pool of her own blood. Kain rushed over to her. He burst into tears at the horrifying sight. He grabbed the necktie that was in her pocket and ran off into the distance. It wasn't until about a year later that he came back.
On his birthday, he returned to Midgard. That entire year, he was in a depression and managed to gain up enough courage to go back to Midgard. A tall man, similar to Vincent, greeted Kain and told him he was the new Captain. Kain asked for his name. Captain said that he had gone by many names and many identities to keep Hunters safe. Kain asked for his real name. Before the Captain walked off, he said three words. “Call me Richie.” He walked off just like Vincent years ago. Kain walked back through the streets of Midgard, greeted by warm smiles and hugs from everyone. It was nice until he got home.
Niak was waiting with a variety of weapons and a whip on the table. Kain ran upstairs with Niak pursuing him with a dagger and the whip. Niak lashed the whip directly at Kain's back, making him wince in pain. Niak kept on doing it for hours, Kain crying from all the pain. After Niak stopped, Kain got up to check the scars. He took off his jacket and shirt and was speechless at the sight. His entire back was bleeding and some of the whipping scars were deep. Kain never wanted to tell anyone about this. Before Kain could get his jacket back on, Niak threw a dagger at Kain, hitting him in the arm. Kain removed the dagger as his arm was now bleeding quite a bit. It wasn't as bad as his back, but it still hurt.
Every day, when Kain got home from cruises and Fight Club battles, etc, etc, Niak would beat up Kain in any way possible. Every time, Kain got stronger, but Niak kept on whipping, stabbing, punching and kicking harder every time. Some of the Hunters would get suspicious, but Kain would just lie to cover it up. The only person who has ever seen Kain’s abuse was Athena.
She was visiting Kain when she heard painful screams from his room. Athena rushed upstairs to see Niak whipping Kain. There were drops of blood on the floor and one of Kain's shirts stained completely in it. Before Athena left, Kain told her one important thing. “Don't tell anyone.” Athena walked out of the door. She tries to keep an eye on Kain during cruises, battles, etc, and talks through what is happening at home. Athena is the only person Kain could trust with this information, but now you all know. Warn the other Hunters about Niak and help Kain the best you can. I have tried my best, but he won't talk about it. Now you must take the lead.
I'm sorry this is so sad. I had a rough idea in my head and this is what I came up with. You can come up with your own for Kain or for other Hunters if you want. Parts of the story aren't canon! It's what I have come up with based on canon information and fanart. The only thing I made up without any reason was the Captain's name. Since we all name him after ourselves, he needs a true name so Richie was the first idea I came up with. I've stuck with it ever since. You can use any information from here but if you can, credit me. Thank you!