Erwin is portrayed as serious and calculating. While he trusts his men, he is the first to realize that there is a spy interfering with the Scouting Legion and arranged for his/her capture. He is an eloquent speaker, able to sway many people, such as the war tribunal held to judge Eren and the trainees choosing recruitment branches to join.
Erwin is very grim. He accepts all news, good or bad, mockery or praise, with the same attitude because he knows his objectives and will follow through with them. This trait has likely developed due to the nature of his occupation. He is also a big risk taker, gambling one odd against an another in a myriad of ways despite the low success rate.
He shares this unique stoic quality with men such as Head Administrator Dot Pixis and Generalissimo Dallis Zacklay. As Armin puts it, these men have the ability to send their men to die if it will further the cause of humanity. They will sacrifice their own humanity in order to bring change to a world where those who cannot sacrifice the important things in life will not change anything.
He is later formally introduced when the Scouting Legion is clearing a town inside Wall Maria. They discover Titans are moving north, towards the Trost District of Wall Rose. Fearing a breach, they head to Trost District and narrowly save a trapped Eren, Armin, and Mikasa after their mission to block the hole is complete.
Erwin later on explains the upheaval Eren has caused in the politics of humanity. Later, he successfully gets Eren recruited into the Scouting Legion and arranges for his study by Hanji Zoe, a Titan scientist and squad leader.
At the recruitment drive, his speech about the future of humanity convinces most of the top 10 trainees from Eren's unit to join the Scouting Legion. However his same speech has discouraged many more due to the grizzly nature of the Scouting Legions' various fatality statistics and failures. In some essence it is a way to weed out the weaker of the recruits with a less resolute constitution.
After the death of the Titan research subjects, Bean and Sawney, he tests everyone in the Scouting Legion by asking them a direct, but confusing question to gauge their reactions. When this turns everyone innocent, he begins planning the next expedition to capture the spy, guessing they have abilities similar to Eren and are intent on sabotaging the expedition.
However the plan fails as the Female Titan manages to escape but only after inflicting massive casualties on the Scouting Legion party. Erwin is forced to call a retreat back to the Wall's safety.
For the costly failure of this mission, Erwin is blamed for the fiasco and summoned to the imperial capital to explain. However, he trusts Armin's gamble that Annie Leonhardt is the Titan and orders her covert capture en route to the meeting. He briefly explains this to Nile Dawk.
She is eventually apprehended and Erwin goes into an emergency meeting held to sort out the day's events. He manages to convince the council that despite the expedition failure and subsequent death of some civilians in the inner city, they finally catch the Female Titan successfully and this indicates that there must be more of her kind hiding among them.
He is still there when Wall Rose is supposedly breached. It is only after the Colossal and Armored Titans appeared once again and Eren was captured that he leaves the walls in an effort to pursue the Titan Shifters. He and the majority of the Scouting Legion follow the hulking Armored Titan and its passengers Ymir and Bertolt with an unconscious Eren, only to be ambushed and attacked by an overwhelming number of Titans soon after. He is injured in the ensuing battle and his right arm is bitten off as a result. His life is not forfeit but he is seriously injured no less. He also claims that he has a replacement for him in the event something happens to himself. It is not yet known who it is.