Klonoa: Moonlight Museum
Despite the titular numbering which might suggest otherwise, Klonoa 2 is actually the third game in the series. Much to the world's surprise Namco released the second game late in 1999 as a WonderSwan cartridge, where it promptly sank into ignominy - at least in the states, anyway, where the WonderSwan exists primarily as a subject of occasional "Is it ever coming to America?" missives to IGN's Q&A section.
Most import shops list Moonlight Museum as a puzzle game, which suggests that most import shops have not actually tried playing the game. It is in fact simply a fully-2D semi-sequel to Klonoa. I can't give any particulars on the story seeing as it's all in Japanese and I long ago abandoned my pretenses of learning the language, but it costars Huepow from the PlayStation game, suggesting it's either a prequel to the original game or else abides by the tried-and-true handheld standard of ignoring continuity altogether.
Visually, the game falls quite a ways short of the unique and breathtaking graphics of its console-based brethren. The 3D backgrounds and intricate level designs are replaced by what appears to be an exercise in showing exactly how badly the WonderSwan's passive-matrix screen is capable of blurring. The reduction in dimensions is compensated for with some very clever action puzzles, but there's no story to speak of, nor are there bosses every other level. So basically you have a straightforward action-puzzle game with Klonoa's distinct, simple play style and the annoying changing orientation (between landscape and portrait) that is apparently a contractual obligation for all WonderSwan games.
Moonlight Museum is a solid enough platformer by any standard, but considering the utter dearth of playable material on WonderSwan it's practically a system seller. Which doesn't mean I'd recommend buying a WonderSwan just for the sake of playing the game, but you could certainly do much worse - almost any title on GameBoy Color, for instance.