O₂ Engine – HTML5 Visual Novel Engine
o2engine.org
o2engine.org
There exist a variety of other visual novel engines that are more open, including the HTML5-based MIT-licensed TyranoScript[1] and the Japanese-only not-quite-as-open Almight.jp[2]. If you're not in need of HTML5, Ren'Py is probably the most popular VN-engine outside of Japan, and is also available under the MIT-license[3].
In addition, as far as I can tell the "NovelSphere" site seems to be Japanese only, which I would imagine is a major roadblock if I want to release an English-language game.
edit: I hope this didn't come across as too negative, changed a bit to clarify.
In fact, they lost a court battle recently to Hutchinson 3G over a 'bubbles' trademark that O2 holds.
http://news.bbc.co.uk/1/hi/business/7450513.stm
The relevant portion of the trademark (https://inventively.com/search/trademarks/78618164) is as follows:
interactive entertainment services, namely, providing online electronic games services by means of any communications network;I'd put a full KAG3 code example somewhere on the page (rather than the tiny picture). I hadn't heard about KAG3 and I have no idea how to write it.
Good luck!
There's some old English documentation on http://kirikirikag.sourceforge.net/contents/index.html (which seems to be a translation of the original upstream Japanese version).
Just like mook said, O₂ Engine is just a Kirikiri-compatible engine written in JS, so the site he introduced will provide you with useful information.
Anyway, we'll put sample scripts later!