Levi didn't have to do much to improve the relationships between the different races of the residents of Darkin City. Being located within the dangerous mountain range seemed to be motivation enough for everyone to put their differences aside in order to work together for the betterment of their lives.
Moreover, being able to witness the prowess of a Great Demon Lord when Tyler had erected a magic barrier across the entire city was enough to fill the newcomers with awe and reverence for their new sovereign.
This was because only large and major cities could sustain a city wide magic barrier, not something Freemen like them had experienced before.
Most of them lived their lives constantly on high alert, never knowing when they would be attacked by a beast stampede, or when they would need to pack up and move to survive.