Get the "secret"characters by going after them in the same order that they're listed on the website.
EDIT: Don't know what to tell you guys except that it worked for me. You have to keep choosing to replay until you've gotten everybody, and then the sword girl will be last.
Also it doesn't actually have to be in the exact order, but you have to do them in sets.
NW building: Basketball girl - Wrestler - Gymnastics -> Locker room scene SE building: Blond paizuri scene - donut girl - girl with necklace -> blond twintails girl Both buildings: Glasses teacher - white-haired teacher - lab coat teacher -> yakuza woman Then knock out the last two (delinquent girl and makeup girl) and at the end of the day you'll get the sword girl.
To add to Seyser Koze's helpful comment. After completing each set you'll be asked two question one after the other. Choose "yes" for first question and "no" for the second question. Complete all sets to get the Bamboo sword chick at the end.