Yûri (also spelled Yuuri - it's a long U) shows up somewhere in Yuna 2: Eternal Princess for PC-Engine. Apparently Yuna's class went on a field trip to the moon (this is the 30th century after all). Presumably Yuna wandered into some ancient ruins, triggering the defense system - a single Guardian Unit. The rest is a mystery, contained in Yuna 2.

Yûri eats a LOT. It's possible she has a power plant similar to "Mr. Fusion" from Back to the Future Part III in that she has to intake organic matter to generate energy. Then why could she function on the Moon? Err... maybe there's a battery charger there, or maybe she also has a main power plant like the Transposition Engines from Nadesico where it uses the force of the vacuum of space as its power source.

One interesting note about Yûri: in the two Anime OAVs her weapon is a sort of flattened staff that she creates from the balls on her shoulders, but in Yuna 3 she turns her hands into guns or swords; her weapon is listed as "Twin Dragon Fangs."

The question is: who is the ancient technological empire that left Yûri behind? Maybe the same ancient civilization that created the Mechanizans? One possible clue is the symbol on Yûri's outfit, like a U with a tongue and a ball inside. A very similar symbol appears on Ryudia's armor, and according to one document I've seen her birthdate is 2 BC; could she be a remaining member of the ancient civilization?

Yûri's ribbon	Ryudia's armor

Sound Clip (WAV) Ootoro raisu baaga... made tabe owatte nakatta no ni, doko ka ni otooshita n'da desuuu! My Ootoro Rice Burger... I lost it somewhere before I was done eating it, I did!