Calliope Nightingale.
Elias walked on the hallways of the academy, his mind occupied. It only dawned on him when the woman introduced herself.
How could he forget?
Well, it was simple.
Calliope Nightingale was not really a main character. She was just a side character, and one that had very few appearances.
She gradually became forgotten as the story progressed, but later on she came back to the story as the "Queen of Violet Storms".
Her character… he originally wrote her as a prototype "older friend" character to help Rain during the early phases of the academy arc. Him
Although that idea didn't really come into fruition, since when he was writing her he was struck with the realization that perhaps he was making too many female characters.
And they were awfully close to the main character as well.
[1] - Au Revoir Club owns the cafe that Elias and Hajin went to way earlier.
[2] - Felix deal contains a laboratory.