"Yuka System" is a visual novel engine used by a few companies such as HookSoft, Smee, PeasSoft and feng.
Unfortunately little to no information about the engine (and therefore no documentation) is publicly available, so I took a few weeks to carefully analyze the game archives.
Why? Because we (Rabu Rabu Translations) are currently working on a fan-tl of the game "Lover Able" by Smee: http://raburabu.net/projects/lover-able/progress/
I've attached my notes on the file formats and some general information I've gathered in the doc/ folder.

Yukatool aims to be the all in one solution for everything neccessary to properly work with the engine.
This includes features to deal with the three main components: archives, graphics and scripts.
At this point, the following operations (tasks) are supported:

• help Displays a list of available commands
• unpack Unpacks a yuka archive (.ykc) to a directory tree
• pack Packs a directory tree into a yuka archive (.ykc)
• patch Copies all files from one archive to another
• split Splits a binary yuka script (.yks) into it's main sectors (code, index, data) and generates a comprehensible breakdown of the former two.
• decompile Decompiles a binary yuka script into a yuka source file (.ykd) and an optional string table (.csv). Works on entire directory trees
• compile Compiles a yuka source file (.ykd) and an optional string table (.csv) into a binary yuka script (.yks). Works on entire directory trees
• unwrap Converts a yuka graphics file (.ykg) to png and optional meta information (.meta)
• wrap Converts a png file and optional meta information (.meta) to a yuka graphics file (.ykg)

The following tasks are planned for future versions:

• disassemble Unpacks a yuka archive and automatically decompiles all scripts / unwraps all graphics
• assemble Packs a directory tree into a yuka archive and automatically compiles all scripts / wraps all graphics

Yukatool is completely command-line operated. The basic syntax is as follows:
yuka.exe [flags] <task> <arguments...>
Flags do not have to be at the beginning and can be specified in a long or short form:
-rvw adds the flags --raw, --verbose and --wait
Which flags are available depends on the task you want to execute.

If you need technical support, want to contribute or have any other questions regarding the project, feel free to contact me at [email protected] :)