Well, some preliminary results. I tested Micaiah, Leonardo, Edward, and Volug, since it happened to be easy for those people based on the saves that I had. Something that is common for everyone so far: starting from dead-even Normal bio (aka right on the center line), it takes exactly 25 turns to return to that spot again.
With 1 being Worst, 2 being Bad, 3 being Normal, 4 being Good, and 5 being Best:
3345555555543321111111123 - Micaiah
3345555555543321111111123 - Volug
3343332233443322334433323 - Leonardo
3555311134554211235553111 - Edward
You can see that Micaiah and Volug have identical biorhythm waves, which is expected. I have a feeling that all characters in the game will follow the same 25-turn cycle, and t's just a matter of finding distinct Bio waves and mapping them to values. Then you can just put it into chart form, and voila. It requires either a dead-center Normal or a couple of data points to tell exactly where on the bio wave a character actually is.
I'm not sure how useful that sort of information will actually be, though. But anyway, there you go. Just 70 more characters to go, right? ;)