Erwin's father was the father of Erwin Smith. It was his words that motivated Erwin to try and prove that his theory about the Walls and the world beyond was true.
Erwin's father was a tall, lean old man with short light-colored hair, circular glasses, and an unkempt beard and mustache. His hair was kept parted on the left side and his face showed distinct cheekbones and wrinkles. Erwin's father wore a white shirt with the sleeves rolled up and dark gray pants when teaching his class.
Erwin's father appeared to be highly intelligent, considering how he theorized about the Walls. He also cared deeply for his son's future, so much that he did not reveal his "dangerous" theory in his history class for his sake and the students', but later told Erwin.
Appearance: Erwin's father was a tall, lean old man with short light-colored hair, circular glasses, and an unkempt beard and mustache. His hair was kept parted on the left side and his face showed distinct cheekbones and wrinkles.
Personality: Erwin's father appeared to be highly intelligent, considering how he theorized about the Walls. He also cared deeply for his son's future, so much that he did not reveal his "dangerous" theory in his history class for his sake and the students', but later told Erwin.
History: In Erwin's youth, Erwin's father was a teacher in their hometown. One day he was teaching history when a young, bright-eyed Erwin Smith asked him a question.
How Erwin's father echoes later: When Nile Dok is speaking to the other Military leaders about his past with Erwin, he reveals that the question he asked his father that led to his subsequent death and Erwin's dreams of knowledge about the outside world was "How did they find out that there aren't any humans outside the Walls?."
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.