Erwin's father

 was the biological 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.

Appearance
Mr. Smith was a tall, thin old man with short blond hair, circular glasses, and an unkempt beard and mustache. He had wrinkles on his face and wore a white shirt with the sleeves rolled up and dark gray pants when teaching his class.

Personality
Mr. Smith 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, Mr. Smith was a teacher in their home town. One day he was teaching history when a young, bright eyed Erwin Smith asked him a question. He vaguely avoided it, ending class soon after. When they make their way home, he sits Erwin down and gives him an honest answer to his question based on what he believed. He mentions that the history books, distributed by the government, were full of mysteries and contradictions. He believed that it was near impossible for humanity not to have passed down an oral history. Instead, the only way to insure that humanity forgot about the history before the Walls would be to alter their memory in some way. Due to Erwin's naivety, the Military Police caught wind of Mr. Smith's theory. He was then secretly detained and tortured for what he knew. His body was then dumped in a far off town, made to look like an accident. Considering that he died before Erwin Smith was old enough to enter into the Survey Corps, it would seem that he lived long before Eren Yeager was born.

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?".

Relationships

 * Erwin Smith - Mr. Smith trusted his son so much that he revealed his dangerous theory about the Walls to him. But Erwin ended up telling other children about this theory and the Military Police caught wind of it which led to his father's death.

Attack on Titan Anthology
Mr. Smith appears as a minor character in the short story "Skies Above" in the Attack on Titan Anthology. At the school where he works, he is acquainted with fellow teachers Rene Browning and Lyla Remus. When he posts the year's grades for his class, Lyla is surprised to see that Erwin is not at the top of the class, to which Mr. Smith replies that he wants to give his son something to aim for. Some time later, Rene witnesses Djel Sannes arrive at Mr. Smith's office to take him away. She grows worried for him, but Mr. Smith assures her that everything is fine.

Trivia

 * Mr. Smith is one of five people believed to be tortured and killed by Djel Sannes and the Military Police Brigade.