New secret discovered.
After finishing story mode once, play it again. The story will have changed as new parts are added (such as a Midori vs. Fumi fight after the Nao/Natsuki fight).
Edit... heh heh... Instead of a Yukino/Shizuru fight, it's Haruka/Shizuru! Unfortunately, it's a fight where you are intended to lose. Anyway, her "Child" is her throwing her arm-band.
Unlocking Arika.
Play story mode twice.
Arika does not have a Child, instead she dances around, draining the opponents HiME gage to zero.
(Note: You cannot play as Arika in BR.)
Unlocking Haruka
Beat Dark Mikoto in BR while playing as Yukino.
Win 14 matches in Survival while playing as Yukino.
Play as Yukino six times in Free Battle.
(Note: You cannot play as Haruka in BR.)
Unlocking the swimsuits
Play as or against a certain character 6 times in Free Battle mode.
And for some reason, when I unlocked Haruka, I also unlock "Super-Duran" Natsuki also.
This is what tipped me off on how to unlock Arika and Haruka.
Top half is for Bakuretsu?!, lower half is for Senretsu!!
Front page of above site.