Arabella’s magic is just enough for her to survive in the wizarding world, but if she had been born as a witch, she would definitely have been able to cast a Patronus. The swan may look like a graceful, harmless bird, but its looks are deceiving. It can be a vicious, unrelenting beast if provoked; make Ara angry, and you will face the bitter consequences. Additionally, swans mate for life. There will never be anyone for Arabella besides Charlie.