@saifors @Dezaki @Scrabbleman @HDMI1
Actually, it seems as though the manga skipped his backstory. Megrez was supposed to ask Sei about why he goes about things the way he does, to which Sei explained his backstory (or rather, his father's). I too really like this spin on the "supposed to be generic isekai protagonist."
I just now checked the raws for the next few chapters (since it's up to chapter 26 in Japanese) and the backstory has indeed been skipped. So, since the explanation for it was skipped, I'll explain it here:
In short, his father's a policeman, but he ended up catching someone who seemed like the culprit, and the father was convinced of it at the time based on the evidences and didn't question it more. The person was judged guilty and incarcerated, but then the real culprit was found. Thing is, by that point, the innocent who was punished for the crime had already committed suicide.
When the father learned of it, he immediately started blaming himself for having led to the death of an innocent. He became quite depressed and alcoholic as a result. That happened when Sei was quite young, and the change in his father's psyche as a result of all that, along with his father repeated words to his son to never make the same mistakes he did, all had a big impact on Sei's viewpoint of things as he grew up. That's the reason he always questions things. It's been drilled into his head by his father almost his entire life.
Prior to that having happened, his father was his idol/hero-figure. Sei wanted to become a policeman too, but then that happened and the idol crashed and became an example of what not to do for both of them.
Edit:
@RayKadoodles yeah, that's pretty much what happened.