Spoiler ahead, because, obviously spoiler:
Raiko: it's episode 2 content, you need to go with her to the park, and then go with her to the swimsuit shop. For all gallery you need some options tryouts and also have tons of relationship points
Koko: you need to go with her in the hotel for the kiss challenge instead of Raiko
Serena: uhhh you need to kiss her on the roof and also have enough relationship points with her