Attack on Titan 2. Books 3 and 4

The world of humans trembles under the crushing power of the Titans. Sacrificing their freedom, humanity has taken refuge in cities surrounded by high walls, hoping to protect the survivors. But once again, the Titans have managed to break through the wall and invade the city. Many have died fighting the monsters, including Mikasa's best friend, Eren.
Stunned by the news of his death, Mikasa loses her vigilance and finds herself in mortal danger, but suddenly a mysterious titan appears to her aid, attacking only other titans. Who is her enigmatic savior?