Many side characters has some sort of content right now with some interactions when looking at social profile list: Gia, Liz, Clara, Kaede, Shizuka, and Sophia. Sarah, Bonnie, and Luna were mentioned in some content in this report.
All the characters you can at least interact with and talk to. The only exception is Felicia as she only appears during story events at the moment.
The point of introducing them is to tell the story. Every single character has at least appeared in another character’s story like Kendra and Helena in Sasha’s story or silver, Iris, and Sarah in Prim’s story.
The are all important in some way in telling the story of Argleton as they help the individual main girls stories like how Iris is Prim’s cousin or how Clara helped Yui.
I am not the dev but the game is ongoing and reading the progress reports explain how these things work