Nicholas Nesterev was born to Marilyn Nesterov and Mikael Mikaelson, the result of both of them having an affair on their husband and wife.
Born a few days before Kol Mikaelson, Nicholas took his mother's last name due to the fact Mikael didn't want his wife to know he had an affair as he had not yet known she had one of her own.
Raised as the son of Marilyn and Asmodeus Nesterov, Nicholas occasionally played and interacted with his paternal half siblings when Mikael took his kids with Esther hunting.
After Henrik died, Mikael demanded Esther include Nicholas in the immortality spell as he refused to lose anymore of his children.
When Nicholas discovered who his father was, and after he completed his transition, he left his siblings as well as those who weren't his siblings behind and traveled on, always avoiding them until he met Henrik as Hunter and officially called him his brother, assuring him he didn't blame him for what Mikael and Esther did.
They traveled together from then on, avoiding their siblings Hunter decided to go stalk their siblings for a while and Nicholas met Julian, who introduced him to Lily, who introduced them to the heretics.
He became fast friends with all of them, and considered them their siblings up until they got locked in the prison world.
Nicholas then decided to reunite with Hunter, and they remained together, avoiding their siblings until they arrived in Mystic Falls.
Sorry it's short, still trying to clean it up some, I haven't even written about his history with Katherine yet