This is Tensei Ryoushu no Yuuryou Kaitaku: Zense no Kioku wo Ikashite White ni Tsutometara, Yuunou na Jinzai ga Atsumari Sugimashita.
I was working for a black company* when I collapsed from exhaustion and woke up to find that I had been reincarnated as the son of a weak lord in another world. I was attacked by a demon, lost my parents, and took over a territory that no longer had any residents... The path I chose to survive was to gather people under extremely white conditions!
This is a territory management fantasy that starts with no residents!
After his parents die from a monster attack, Lyle Alenz tries to strengthen his inherited territory by recruiting people. He used to work for a terrible exploitative company in his previous life, and tried to use extremely favorable conditions, but the other residents of the world are used to being exploited and didn't believe it.
One person does accept it; an S-ranked female knight
The cover of the first volume shows the knight and who I assume are the demon lord and saint(ess) from the question.
*: The term Black Company refers to exploitative work places in Japan.