Once trained, you battle monsters. Losing battles results in "breeding" scenes, which are a primary source of Charms —special items with unique effects used to customize your Oni’s abilities.
@m0rph3us_void remains unidentified. What little we know comes from metadata embedded in the v1.04 executable (a 212KB .exe file that also runs under Wine and, oddly, on a stock PlayStation 2 via the Linux kit). The metadata includes a single string: built: 2003-02-29 . February 29, 2003, did not exist. A leap year error, or deliberate? Onigotchi -v1.04- -BadColor-
While "Onigotchi" is likely a play on words combining "Oni" (Japanese for demon/ogre) and "Tamagotchi," this specific string is frequently associated with: Custom Game Versions Once trained, you battle monsters