Poorly written and try to pretend it is natural and "positive" to be a slave (Seriously, who would purposely ask to be reduced to slavery. It happened, people who lived their entire life as slaves would be unable to live free but it is not the case here.)
Her reaction should be very different. Either she decides to trust him (for no reason except that he saved here once) a little bit like Deku from My Hero Academia when he met that mind control guy or she would be wary of his power.
On top of that, it has all the flaws of this genre. "Heroes" are unidimensional bad guys, Main Character is OP or useful yet nobody seems to notice until he was kicked (Not necessarily bad but it is not very subtle here). Of course, he immediately meets a pretty girl who is going to fawn over him and praise him night and day. Of course, he barely has a personality yet everybody loves/admires him, braindead bullies to beat up to show off, etc.
His power is neat though I don't expect much from it. It doesn't seem like the author intends to be creative when its work starts with EVERY trope you can imagine.
Personal taste: I hate how "Video Game" or "Modern" these medieval worlds are. From an unrealistic ranking system to modern clothing and silverware. These worlds hardly manage to feel real, they are beyond unrealistic (as in "They make no sense". Universes like Lord of the Ring, My Hero Academia, D.Gray-man or Nukenai Seiken-chan feel real despite the Dragons, the humour, etc.). Everything is so "clean", your activity is your CLASS as if you were in a video game and everything about it is "recorded". Like "You're a beast tamer" and "Beast Tamer can't do much", "You're a Level 3 Mage" so "You can use X & Y spells", "You are a Rank E adventurer" so "You are THIS weak and can only do that". I wish they would put some "love" into their worlds, try hard to make them believable and interesting instead of throwing mindlessly every trope and include some distinguishing feature without any care. You can't even blame "Games" for that, Goblin Slayer managed to create a believable world despite being heavily inspired by Dungeons & Dragons.
Finally, I wished their world were a little bit more "ruthless", they are too kind caring about the weak and trying to protect everybody. Why would you forbid someone to try to kill monsters simply because their rank is too low? Let them go (and die if they are too weak). Give people leeway and responsibilities.
Sorry for my rant.