Eren Yeager grows more than any other character throughout Attack On Titan, being shaped by countless hardships. He's nearly killed while rescuing Mikasa as a child, and then later is cut down by her to end the Rumbling. During these two events, Eren meets many people and is exposed to many new situations that test his worldview, inner resolve, and humanity. By the end of it, Eren doesn't see the world outside the Walls as something worth saving, and wipes out 80% of humanity.
Source：https://www.cbr.com/attack-on-titan-10-characters-who-helped-eren-grow-how/
