Something that always intrigues me about them is their world. We know it's the future, but just how far into the future is it?
Interesting stuff in their world:
-Synthetic food are eaten regularly
-Regular trips to the moon
-Natural vegetation is rare
-Supposedly, the Grand Unified Theory is solved
The lack of natural vegetation makes me think that Renko and Merry's outside world is the same as Seihou, or at least near the era. It also helps that many Seihou tracks get remixed by ZUN and put on the music CDs. Alternatively, they could come from Yumemi's world where the theory was also solved. Anyway, just a thought.