Karma will always be real
A wealthy duke's wife gave birth to twins, Mika and Mirkon, they were the complete opposite. Mirkon was calm, and stoic, while Mika was playful and mischievous. As both of them grew, they noticed many nobles beating up commoners, or commoners stealing from the nobles. Mirkon thought that it was normal, but Mika thought it was unfair and decided to take things in her own way.
During lunch, she suffocated a bully in the Restroom. There, she felt satisfaction she never felt before, and began to murder each bullies day by day. Some time later, she was caught trying to hurt the rude teacher, and was sent to the principle's office..
Long story short, the Duke was mad and didn't listen to a word Mika said. With no mercy, the Duke experimented on Mika as punishment, and accidentally giving her the ability of immortality and dangerous uncovered powers.