Yeah...the manga have several problems with logic, but who read enough shoujo is used to it.
Humans and werid heads seems to be the same from the neck down.
Is absurd for humans to be treated like pet when they are able to do everything a inhabitant of the isekai can, th eonly difference is literally just the head, at most they should be seens as dimensional sliders or aliens, for their status to be pushed to possible pets...is just crazy.
The heroine is the only one that can't speak for trauma, other isekaied function like normal.
Even if normal humans are rare and possibly collectors want them making them pets instead of granting citizen right make no sense whatsoever.
I'm guessing that was he bully co-worker or boss, the reason for his isekai is likely a incident or maybe suicide after they pushed the disappearance of the heroine on him after workplace bullying.
He will probably meet heroine when blackhead brings her in the office for some paperwork, and it will worsen the trauma until balckhead scares the shit out of him.
Last thing, is super stupid that people of the isekai define themselves as having orrendous heads.