She swam up from the Thames to attend the meeting as a representative of the mermaids. She was hauled out by pulleys and placed in a seashell in a large glass aquarium, or tank. A charming mermaid, according to Magnus Bane, Arabella was among the attending Downworlders who spoke up about their disdain in the direction of the meeting, surprising the Nephilim. During the awkward meal that followed, Arabella asked for a scone.
At the next meeting, Arabella was the only Downworlder who responded to Amalia Morgenstern's attempt to pacify the arguing sides by offering scones; Arabella asked for scones and cucumber sandwiches. After the meeting, when Arabella's tank was being carted out of the Institute, Arabella was frightened by the attacking demons and hid at the bottom of the tank. Granville Fairchild and Josiah Wayland protected her, standing in front of her tank to fight off the demons.
- She is the first mermaid to ever be known by name in the series.
- Vampires, Scones, and Edmund Herondale (first and only appearance)