"Hey I'm MC, I live in a world where slaves are common in every town and city, slaves are known by the huge metalic collar around their necks. One day my childhood friend showed up with a huge bulky looking cloth thing around her neck that she NEVER takes off, even when I'm basically dating her she still never takes it off, but I'm a dumbshit so I don't ask her about it or wonder if there might be something going on."
Also him falling so hard for this girl that is constantly doing something against her own will (aka honey trapping him) that he actually puts all his faith in humanity on this one girl is...
And what's even the motivation for the big bad here, is the MC some prince or noble since the girl was a maid and he has enemies from there? If so he should have been aware of something like this from basic nobles education.
Or did the sword master guy do all this because he "sensed" MC would be a good deciple and needed to ruin him to build him up and some bullshit? That this is the most likely plot is CRAZY levels of dumb right?
Other options are limited to things like godly power and time travel, or maybe someone just seeing him on the street and thinking FUCK THAT GUY... So just no. Also this bad guy has to know the girl is his childhood friend and somehow aquire her (and doll her up to attract the MC) from who knows where (the past!?).
And why did MC never find out what happened to the girl that went basically missing? Sure he was a child but isn't that basically MORE reason for him to ask people about it? Imagine a kid having their first crush or best friend just suddenly move away and then NOT asking anyone about it? Thats just no way right.
I mean yes maybe she just moved away, but even if that was the case how did all their conversations go after meeting back up? Did the big bad give her super slave commands where he gave her a huge long backstory to tell MC when he asks about her life? And some super plan on how to avoid suspicion on the collar and also some premade orders to make her go meet whenever new orders needed to be made since there's no way everything was planned before the first reunion up to including the pushing.
How goddamn easy are these commands, is it enough to say intention and slaves will do their absolute best to live up to that by their own planning and intellect? What even is a person in this world and why hasn't it evolved to a system where only the king isn't someones slave? Imagine the work efficiency and public order if everyone was always completely focused on only their ordered tasks and no wants or free will, yes it's awful but damn efficient. So efficient that there's no way society would NOT go that route if such convinient slave magic existed.