@Kaarme Whether Sakagami's personality has a flaw is rather a separate issue--after all, he is not in fact going for the new girl; I was just pointing out that there would be nothing wrong with doing that. Kaori is distant and formal (even when she's insisting on informality!), and it's hard to get a handle on what she's thinking. Maybe she's just shy, but in such a way that it's hard for someone outside her head to figure out. That doesn't make her personality
bad, but if you're making a comparison, the new kid is cheerful and seems straightforward, easier to deal with. In any case it was just a quick throwaway line meant to amuse, I wasn't doing deep character analysis.
But I don't actually think your assessment of Sakagami is very fair. Having fun being a mentor is not a character flaw. And he doesn't treat Kaori coldly--he insists on treating her with
respect because he sees her as superior. Which she is--experience aside, she's clearly smarter and more competent than he is. I'd say it's actually more plausible to say that if he insisted on talking to her like someone younger/inferior even though she's got more seniority at the job and is better at it than he is, that
that would reflect inability to stomach a younger person being better at their work--it would be belittling her and ignoring her competence. So he's damned either way.