To Unlock Them, You have to play on Festival Mode.

For Mikoto Cat, You have to Unlock her By Playing on Challenge Mode

Effect - Code

Arika 1.5 - Finish Festival With Ultimate Nina
Arika 2.0 - Finish Festival With Lena
Lena - Finish Festival With Miyu
Mai - Finish Festival With Natsuki
Mikoto Cat (X) - Collect 2000 Coins In Challenge Mode and Buy the last Figure
Miyu - Finish Festival With Shizuru
Select Arika 1.5 - After you unlock her, Push Select Button in Character Screen
Ultimate Nina - Finish Festival With Anyone after you have finish Arika 2.0 Festival