Marinette and Damien Al Ghul-Wayne were twins. They were the best assassins in the league. Ra's was obsessed with Damien and told Talia (the twins mother) to get rid of Marinette because he thought she was useless.
But Talia didn't take any of the bullshit and started training Marinette harder than Damien so she could get through any obstacles that Ra sent to kill her.
Marinette is sweet, caring and bubbly but harm her loved ones she'll be deadly in a matter of seconds. While Damien acts cold and emotionless but cares for his loved ones especially Marinette. Annoy him you'll be in the hospital for broken bones.
They both were very powerful that they can beat highly trained grown up assassins. While fighting Damien fought without plans and strategies but won due to on spot actions. While Marinette knew what the opponent will do so she is always wins the more reason for Ra to kill her as he thinks her as a threat.
One day, their enemy Deathstroke arrived and Ra died in the fire. Damien was taken to his father Bruce Wayne, while Marinette was taken to Talia's trusted assassins Sabine and Tom Dupain-Cheng in Paris who raised her like their own. But neither Marinette nor Damien knew the other was alive or dead.