Hello, and thanks for playing!
I realized that part is still a little confusing so I'll make it more clear in the next update.
Basically, there are a sequence of battles in which you have to form a team composed of the Myth member that tells you she wants to stay (in the case of Calli that would be Kiara) and the other 4 members have to be non-Myth (so basically, Kiara + JP or ID girls)
This sequence is repeated a couple of times. Also, be sure the 5th is also none of the Myth girls since the 5th participates in the battle as well!