If you consider .NET applications (which require the .NET framework) portable, you can consider Ketarin to be portable as well. It doesn't need an installation, so you can run it from any location you like.
By default, Ketarin will create a database file in %appdata%/Ketarin, so that it is a per-user database. You can also move this file to the application path (where Ketarin.exe is located), then it will always use the database from the startup location and is thus suited for USB sticks or the like.
At last, you can use the command line argument /database=FILENAME to start Ketarin with the specified database.
Remember to enclose FILENAME with double quotes when its path contains spaces.
Note: Since version 1.5 Ketarin also stores relative file path information, so changing the drive letters will no longer cause Ketarin to forget where files are located.