It's a character that has to do with the graffitti around the main map (not in the echo chamber of Phoebe that was recently added). I'm not sure how much you've progressed the character's storyline, so I'll leave it at that. Try messing with the graffiti if you haven't already. I believe it should be obtainable by simply completing her storyline? She was one of the earliest characters, and I first played a year ago at this point so memory is a bit hazy. If not obtainable by completing her storyline, her echoes will probably become available on the map after completing her story. I don't see them on the map though, so probably just completing the story is enough
Hope this helps :)