"A simple life with my unobtrusive girlfriend" sounds so much more... natural, it rolls off the tongue better, it's pretty accurate still. Versus just "A simple life with my unobtrusive girl". Referring to your girlfriend as "my girl" is perfectly normal and doesn't sound bad in any way, on its own. But in this case, with the extra stuff added on, it doesn't flow. Just my suggestion, when it comes to the english title. Idk if the non-english title is changed or anything. That's all.
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I want to add that I forgot Kanojo with the exact same kanji can mean both girl and girlfriend in Japanese so maybe that's why its translated to girl and not girlfriend... Koibito is common as well to refer to one's bf or gf, as it just means lover. (Love+Person) but it's just lost in translation here. I urge you to fix it to be "girlfriend". It has nothing to do with the gameplay itself, I understand, but I see it as a necessary change and I'm sure many others would as well. HOWEVER, it will be really de-immersifying and feel very off btw, if "girl" is used in weird places in an unnatural way though. Like in place of wherever he's referred to his "sister" pre-censoring.