No problem. I'm explaining it for FE10 though, not FE9.
As we all know, every character in RD has a biorhythm (the sine thingy). For playable characters, there are eight different patterns. that are either more erratic or subtle (Rhys and Tanith have a very subtle biorhythm in comparison with Heather and Boyd, who have a very erratic one, for example). And as we also know from what we see in-game, at any given moment, a character's point on the biorhythm is classified as one of the following:
Best
Good
Normal
Bad
Worst
(There are two biorhythm patterns which are exceptions that don't touch the extremities of Best and Worst)
So what does biorhythm do? It impacts a good number of things, actually. Biorhythm affects your hit stat, avoid stat, skill activiation rate, and even a character's chance of finding hidden items.
When a character's biorhythm reads "good," their hit, avoid, and skill activation rates increase by 5 while their chances of obtaining hidden items if they're standing on them increase by 10%. When a character's biorhythm reads "best," the effects are doubled (so the boosts are 10 and 20 respectively). Bad and Worst have the inverse effects of Good and Best, lowering the stats affected, and when it reads normal, their is neither a boost or a deduction to anything.
I wouldn't know how to determine an OC's biorhythm, but I can link you the different biorhythm patterns that characters have:
http://serenesforest.net/fe10/biorhythm.htmlFor those interested in abusing biorhythms:
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Although a biorhythm usually can't be controlled unless you're willing to use a heron's Bliss ability (and forgo giving a unit another turn), for characters you've set up to specialize in getting kills through skill activation there might actually be some merit in using Bliss every so many turns instead of simply refreshing units to move again (it's very situational, however). To get the most out of Bliss, it's best to use it on the characters who have the biorhythm pattern that have the longest "best" and "worst" cycles on their biorhythm (which would be the following: Volug, Rhys, Astrid, Tanith, Brom, Zihark, Aran, Micaiah, Makalov, Leanne).
If you're using any of these characters in conjuction with a combination of skills intended to kill enemies through sheer activation rates, using Bliss will provide the 10% boost to their skill activation rates for more consecutive turns then for other characters.
I'll use Tanith as a Seraphknight for an example:
Let us assume Tanith just promoted to Seraphknight (with no bexp abuse). Her Skill and Speed stats would be at 26 and 26.6 on average respectively (so we'll use 26 and 27 for calculating purposes). As far as Mastery Skills go, Seraphknights have one of the best because Stun's activation rate is the unit's Skill stat (rather than Skill/2, which is what most beorc units have) and it only takes up 25 points of their capacity. Canto takes another 10 points of capacity leaving her with 25 points of capacity for skills left (IIRC) so we'll give her Adept (takes 15 points) and Vantage (the final 10). Both have an activation rate of whatever the speed of their user is (so 27).
If we use Bliss on Tanith, she'll have a 36% chance of activating Stun per attack and a 37% chance for Adept and Cancel. With this setup, if Tanith goes up against something she doubles but 3-shots, she'll still have a very good chance of killing the opponent (or at least because between her two attacks, the chances of either Stun or Adept activating become more and more impressive (and whenever Adept activates, there's another oppurtunity for Stun to activate). I'm not going to bother to calculate the exact odds, but I'd assume they're at least well above 50%. And whenever something attacks her, there's the chance she'll activate Vantage AND Stun or Adept (and then Stun after the Adept activates) to give her the potential to OHKO anything that attacks her. This is as a level 1 Seraphknight. Once she caps Skl and Spd, the odds of anything activating all increase by 10% and between all of her skills and bliss boosts every so many turns, she'll be able to consistently activate one of her skills for easy kills that normally wouldn't be attainable for her.
Yeah, biorhythm has it's uses and abuses.