While the principal job of a good VN editor is line editing (making sure that a line reads well and that a script flows), copy editing is vital as well, and copy editing should follow a style which is consistent both internally and with other comparable texts. That said, most VN editors (myself included) are way too lazy to sit and read the MLA, Chicago, or AP style guide cover to cover and actually internalize it, let alone to extrapolate from them what, if any, changes need to be considered for styling a VN, which, being a different medium, may require different stylistic choices than the media covered by traditional style guides.
As is abundantly obvious if you read older officially-localized VNs, VN style has grown somewhat organically over the past decade and, if you compare against works published in only the last year, you'll find that the predominant style has become fairly consistent across the major localization companies. However, fan translations often miss the mark and make many styling mistakes and deviations from this standard, resulting in irritatingly inconsistent texts.
To help solve that, I put together this brief VN style guide a couple months ago and shared it around a number of people, and I've subsequently refined it a bit in preparation for posting it publicly today.
This is not a full prose style guide by any means, but it covers every interesting and potentially divisive topic I've seen come up in styling VNs; it is, I believe, pretty complete, especially given how concise it is. I've tried to avoid topics of grammar and of style that are not generally deviated from in VNs. Basically, I only tried to tackle areas where people actually have issues. This style guide, I believe, more or less represents the state of the art in officially localized VNs. I haven't read a recent official localization which I noticed to be following different rules than the ones I lay out here.
All that said, take this with a grain of salt: I'm not a professional, and I haven't actually read any official MLA/Chicago/AP style guide cover to cover, though I have dabbled in each of them. At the end of the day, this is more a summary of what I've empirically discovered than anything else. But when you're a fan translation editor, you've got to start somewhere; this is a better option than any other that I know of.
Feedback is most welcome, whether to offer corrections or to ask questions for areas which I have not covered.
Edit: By popular demand, I've made a markdown version of the document so it doesn't display like shit on github. The link has been updated accordingly.
Edit2: who will edit for the editors?