Yuki gets from being a cute and intelligent high school girl to a stealing whore in just a few years. Plausible, i'm not saying it can't happen, but the chances are low and i, as a reader, need an explanation as to how things got here. I need to understand what i read, not just eat the facts. Also, even in that state she would show a bit of remorse, which does not happen. Returning the money later was just a scheme to get more from Miyamoto. And this is how we get to my second example.

Miyamoto is not that bright and doesn't seem to know anything other than climbing. But what he does know, he does it good - kind of how the mangaka makes a manga all about the thing he knows best, climbing. He's also very ambitious and has actual plans to realize his dreams, he gathers money to get to France. Bam! A couple years pass and Yuki becomes a whore while Miyamoto becomes a failure with no real reason. Yuki tells us he never went to France because that's his nature, a slacker that keeps saying he'll do things "Tomorrow". He got into drinking and debt. How in the hell did these things happen? There's no way a change like this is possible, the Miyamoto from highschool and the drunk Miyamoto have completely different personalities, same with Yuki.