There might be a misunderstanding. The title is what Sunao says at the end of chapter one. "So long witch!" is, once again, the localization just like the EN title. So he calls her a bitch with the full meaning of the word, hoping/wishing not to run into her again. It's a quite literal title drop. The resentment i was talking about stems from this (which was promptly forgotten next chapter)
Yes, I remember the title drop, but my point is that it's a double misdirection. See, when Sunao said that, he had a wrong impression about Lemon so it looked like the title is referring to that initial resentment, but then we find out that Lemon isn't what Sunao thought of her. But this is again a misdirection, because Lemon is actually exactly what the title says she is, just not in the sense Sunao originally resented her for.
I just watched the Netflix Ranma reboot and that reminds me that I was wrong about what I said in my previous post. Characters like Lemon used to be very common, but not in the recent era of manga. I guess the best way to put it is this: If you read old Takahashi Rumiko manga from the 1980s like Ranma, Urusei Yatsura, but especially Maison Ikkoku, EVERYONE - every character - is basically some level of eccentric shit person. However, they're written in a cartoonishly endearing way that they're actually funny. Lemon is written like a Maison Ikkoku character. She's the same kind of endearing shit person but given a more realistic take and transplanted into the normal world. That's the best way I can describe it. This manga feels like an updated Maison Ikkoku with fewer gags and more realistic but still endearing shit people.