Volke

Best answer as chosen by Asker / Ratings
Biorythm effects Hit and Dodge ratios, and how often a Master Skill activates.

For example, a third tier unit with a Best Biorythm will have excellent accuracy and evasion, and their Master Skill has a higher chance of activating. Another third tier unit with a Worst Biorythm, however, will have crap accuracy and evasion and a lower chance of their Master Skill activating.