@duderus, I would've preferred what the web novel did rather than what the manga did.
@TanithG, I know right, great point.
@Rayster, I don't recall that being the reasoning in this story, but it's a viable excuse.
@Kuraiaku, I know you didn't say that, I was just bringing it up because it's usually left out of the discussion. Sure, the trope is used for convenience in isekais in a variety of ways. I agree.
@ka-ka-kaitouu, I can respect you went out and deep dived on the meaning of a vassal in middle age/medieval culture and pointed out it's a serious oath. There are two problems with your argument tho, 1) he's technically only a kid in physical form, mentally he's an adult. So he should've offered a better response verbally and in action. 2) Ritsu, a former street urchin, is standing right next to Ars indicating that her hasty generalization about Ars is, at the very least, questionable. Ritsu's presence doesn't mean her reasoning is entirely faulty, but it should've raised some questions and a careful author would've had Ritsu speak up about what he witnessed about Ars. Then it could've gone into some back and forth between Charlotte and Ritsu as to whether she could trust him or if she thought he was a sell out or something.
Her refusing is understandable, doesn't make it rational. Whether she's biter, hurt, or not is moot to the point that Ars' point is factual. He could make her life better. She's choosing to remain in a harsh position simply because of her prejudice. That's really all there is to it. People like to give a free pass when prejudice and generalizations come from those they consider to be oppressed or downtrodden. It's sill prejudice.
(not accusing you of this) The point, faulty ideologies can clog up one's ability to think properly. She's judging him based on a generalization no different than a rich person judges someone of her status based on a generalization. That's kind of the point of the whole store. Ars can see everyone's actual potential rather than simply judging based on outside appearances.
Her being 11 is a good point tho. Just the author wrote her argument as if it was coming from an adult and then wrote Ars as the actual child despite being the adult mentally. I'm not saying she couldn't make a similar argument, but I don't think it would've been worded that way, but then again, her characterization could be a deep thinking one. Bug given that her speech is a "D"..eeeh...I doubt it. I'd give you this point if her argument was more along the lines of "We don't have things we need, most people have things we need, it's unfair." That's what I'd expect from someone 10-12 with a speech rating of "D." ¯\_(ツ)_/¯ I know that's how my youngest daughter frames those types of arguments.
I do think Rayster brought up a good point on this, and I agree there.
The overall problem isn't necessarily her argument, it's that the author made her seem right with Ars' introspection later and verbal agreement that she was right when his very character is an indication that she's wrong. I would've cared less about her argument if Ars had some comeback even if the girl maintained her position. I would've seen her as blinded by her experiences without looking beyond them and respected the MC and author more for creating a scenario where Charlotte joins the team more difficult and well thought out way when compared to Ritsu joining the team.