Heh, nice sig Marshigio.
Now as for Donbe and Hikari, I think some people are being very close-minded here. Nobody complained about Marth or the Ice Climbers or Mr. Game and Watch. They're very well loved. And be honest: how many of you were actually like "who the hell is Pit?" or "Samus is a hot chick?" It's amazing how many people were shocked, and then readily accepting, of these characters.
So give characters like Takamaru, Mach Rider, and even Donbe and Hikari a chance. They might surprise you.
Anyways... Shin Oni Ga Shima is part of the Musashi Bananshi games, which are basically text adventure games that Nintendo released on the NES and later remade for the SNES. Altogether, there is Shin Oni Ga Shima (The New Island of Demons), Yuu Yuu Kii (Journey to the West; the Son Goku story, fyi
), and Time Twist (original). After the success of the Famicom Detective Club games, they went along with these, essentially based on Japanese legends and myth (with the last one being an original story).
Donbe actually grows up by the second half of the story into a really kickass cavedude, and him, Hikari, a dog, a monkey, and a pheasant basically clobber some evil ogress to save Japan. He would definitely be a pretty popular choice for a character, at least for the Japanese. He would just be another "Marth" for us, but he's really cool. He'd fit pretty well into the game.