Arthur (アーサー, Āsā) is a friendly feline who is always biting Suzaku. It is a stray cat which Third Princess Euphemia befriends after she arrives in Area 11. Arthur is later chased throughout the school grounds by the entire student body after wandering into the Lamperouge residence and escaping with Lelouch's Zero mask. The one who caught the cat got a kiss from any member of the student council. Suzaku and Lelouch were the ones that got the cat and got a kiss on the check from Nunnally. The student council adopts him and builds a cat house in the council room. A running gag in the series involves Arthur constantly taking a bite at Suzaku's hand or leg. Despite this typically one-sided relationship, Suzaku takes the feline along with him to Britannia, where Arthur takes a liking to Knights of the Round member Anya Alstreim. When Suzaku returns to Ashford Academy in the second season, he brings Arthur back with him. In the Avalon he was sitting in front of Lelouch and acted like he knows what was being said and agreeing on what Lelouch said. In the series finale, Arthur is shown watching over Suzaku's grave in the middle of the night. Arthur is last seen in Ohgi and Viletta's wedding picture on Anya's head.
Arthur is a male cat. It has black hair and yellow eyes. His skin tone is black.
- In popular medieval history, Arthur is the name of the king who established the seemingly similar "Knights of the Round Table" as opposed to the "Knights of the Round" of the series.
- Arthur is voiced by Michelle Ruff who also does Euphemia li Britannia, Lelouch vi Britannia (child) and Mutsuke Minase. She also voices Rukia Kuchiki in Bleach.