Thanks for the bonus chapter.
What does he actually call her in the untranslated version?
This isn't just about being immune to getting isekai'd by Truck-kun but also durability.
Maybe this is linked to how dense he acts?
Although comedy manga tend to ignore physics anyway, since the truck driver was surprised, we can be sure that he is not normal and can take on actual threats which could be deadly to most. This also explains why she doesn't need a different bodyguard.
I wonder what kind of training he got and if there are more like him...