Eren discovers that the men who Killed Mikasa's parents were sent by a larger group, one that are still killing innocent people, he becomes hellbent on revenge and seeks out the most notorious hitman the underground has to offer, but not to hire him. No, Eren wants to kill the bosses of the organisation himself. To do that, he needs training. he can cut down a titan, but a human is a different matter, especially considering he can't be caught out of fear of execution as an enemy to humanity.
Enter the man they simply call "Death"
The man is swift, efficient, and deadly as a bullet between the eyes.
He's just what Eren needs.
Levi is grieving the loss of his best friend. He feels his control, his patience, slipping and being replaced with that impulsive, kill -first-talk-later attitude that he held in his earlier years out of need to fulfill his oath to Erwin. Hanji is trying desperately, but Levi's guilt is stopping him from listening.
Two things are holding him together, his promise, and the stubborn cadet who refuses to let him fall.
They move their relationship from strictly professional, to that of friends.
But... where does Eren see Levi keep disappearing to during the night?