Arthur

Arthur (アーサー, Āsā) 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 stuck on his head. The student council adopts him and builds a cat house in the council room. A running gag in the series involves Arthur constantly biting Suzaku in various places, yet being very affectionate to virtually everyone else he meets. Despite this one-sided relationship, Suzaku takes the feline along with him to Britannia, where Arthur takes a liking to Knight of the Round Anya Alstreim. When Suzaku returns to Ashford Academy in the second season, he brings Arthur back with him. 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 Villetta's wedding picture on Anya's head.

Source: Wikipedia