The Bandai Wonderswan, later the Wonderswan Color, was a handheld system released only in Japan in 1999, and the version color 1 year later. The system was created to compete with SNK's Neo Geo Pocket Color and Nintendo's Game Boy Color. Though unfortunately it saw little success in competing with both systems and was inevitably faded into obscurity. The system was best known for "RockMan & Forte 2: Challenger from the Future" which was a console exclusive, though even still it did not achieve much success.
Oswan is an emulator that only supports layer removal tools as far as debugging goes.
Mednafen supports many systems, the Wonderswan is no exception. Along with layer removal tools, it also features frame advance as well as a tile viewer.