The traffickers are three unnamed criminals from Attack on Titan. They are already dead at the beginning of the story.
When Mikasa was nine years old, they killed her parents and kidnapped her to sell her as sexual slave. Her mother was considered the only Asian girl alive and the last of her race. That made themboth high value targets to the traffickers.
Later, unable to control his rage, Eren tricked and killed two of the slavers and was almost choked by the third murderer. Both traumatized and scared Mikasa come to realize that the world is a callous and merciless place and kills the trafficker.