She seems to fit the Chloe Chronicle role as Tabane's "daughter" with rather strong implications in the Light Novel/Manga/Anime. In Volume 8 Chapter 4 I personally I don't like the Baka Tsuki translation that she is Laura Bodewig's big sister when she is the younger one. But in the Anime Season Two Episode 12 she's significantly older and rather in the age range to join the main group.
Looking forward to seeing how her character develops in the light novels getting smacked down effortlessly by Chifuyu, while only getting a moment in the Anime that made me think.
Seems late to reply, but if you are talking about the girl in the white dress that Ichika "wakes up" to find standing with her back to him after the failed Silver Gospel mission, that is the representation of Setsura. I am not 100% sure how the system works, but from what I have read and figured out myself, Byakushiki is linked into Ichika's body and has access to his thoughts and conscience (thus the ability of mentally deploying the IS) so it is somehow able to communicate to him this way. The other person who looks like Chifuyu in the White Knight armor is the representation of Byakushiki. This image is possibly generated from Tabane flashing Core 001's "memories" to Byakushiki's core, (hence the fact that both he and Chifuyu have the same One-off ability, Reiraku Byakuyu) so the core remembers Chifuyu being it's pilot thus appearing as The White Knight in Ichika's mind.