Arthur

Arthur (アーサー, Āsā) is a friendly feline who was originally found by Euphemia and was befriended, before joining the Student Council members. Arthur is always biting Suzaku and is usually with him or Anya Alstreim.

First Season
Arthur 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. Milly announced that the one who would catch the cat would get 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 cheek from Nunnally. The student council adopts Arthur and builds a cat house in the council room. A running gag in the series involves the cat constantly taking a bite at Suzaku's hand or leg. Despite her signs of affection to him at that point consisting almost entirely of attacking him, Suzaku takes the feline along with him to Britannia.

Second Season
When Suzaku returns to Ashford Academy in the second season, he brings Arthur back with him. During his time home, Arthur takes a liking to Knights of the Round member Anya Alstreim. In the Avalon, Arthur was sitting in front of Lelouch and seemed to be aware of what was being said and appeared to side with Lelouch. In the series finale, Arthur is shown sitting by Suzaku's grave in the middle of the night. The cat is last seen in Ohgi and Viletta's wedding picture on Anya's head.

Re;surrection
Arthur isn't particularly present in the film, but she is seen on Zero's lap in the video of Ohgi and Viletta's wedding. The manga Re;surrection makes it clear she is currently under the care of Lloyd Asplund, Cecile Croomy, and Nina Einstein.

Audio Drama
In the Code Geass Audio Drama series, Arthur was featured in one of the episodes (episode 6.113 - The Name of a King) where the members of the Student Council had just adopted Arthur and were trying to decide a name for it, but earlier Euphemia had already given the cat the name 'Arthur'.

Some names suggested for Arthur included:
 * Panda (for the black spot around its right eye)
 * Schrodinger
 * Tama
 * J. C. Maxwell
 * Doppler
 * Michelle

The student council decided they were going to name the cat 'Lelouch the Second' and were actually going to go through with that name until Nunnally came in and started talking to the cat. After she announced that the cat's name was Arthur, everyone agrees to it and Suzaku muses how Nunnally and Euphemia gave the cat the same name.

Trivia

 * In Arthurian legend, 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.
 * In Arthurian legend, King Arthur's wife Queen Guinevere cheated on him with their greatest knight, Lancelot. And while Arthur had to be prodded into it, he subsequently went to war against Lancelot. Given that Suzaku's Knightmare is the Lancelot, this could be why the creators chose to make Arthur express initial dislike and later more violent affection towards Suzaku.
 * Arthur is voiced by Michelle Ruff who also does Euphemia li Britannia and Lelouch vi Britannia (child).
 * According to a tweet by the official twitter ,  Arthur is actually female. The dub probably used male pronouns because Japanese pronouns aren't as gender specific (and thus difficult to translate without context) and Arthur is generally a boy's name so they likely just assumed Arthur was a boy cat.
 * Arthur seems to be very aware of what is going on around her and has a knack for trying to distract or otherwise help Suzaku in times of distress.
 * The first two times were likely entirely unintentional on her part, but in talking to Euphemia and being tended by her, she kept Suzaku from confronting her about lying about being followed, and in accidentally stealing Zero's mask, she gave Lelouch the opportunity to publicly cement himself as Suzaku's friend and get him into the Student Council.
 * When Lelouch and Suzaku first argue about the methods of the Black Knights, she starts batting at Suzaku's hair (though that could also just be because he stopped playing with her).
 * When Ashford is claimed by the Black Knights, she attacks Tamaki's hand on his gun, preventing him from shooting Suzaku. This also seems to be when Suzaku realizes she doesn't hate him, because he does smile at her in the Lancelot as she watches it take off again.
 * When the Student Council starts asking him about Zero, she bites his fingers, distracting everyone from the conversation.
 * When Suzaku is watching Lelouch and Rolo at his welcoming party, she's curled up in his arms and biting his fingers.
 * She is snuggling next to Suzaku when he's being self-loathing after letting Kallen beat him up for trying to use Refrain on her, and starts hissing immediately before Suzaku catches a knife thrown by Luciano Bradley.
 * When Lloyd, Cecile, and Nina are discussing upgrades to the Lancelot with Suzaku including a FLEIJA launcher, Arthur is pawing at his leg and then sitting by his feet.
 * Though she was seen sitting at Suzaku's "grave", in the official manga Re;surrection, at Ohgi and Villeta's wedding, Lloyd apparently decided to bring Arthur along and she escaped from Nina to leap into Zero's arms, apparently recognizing him. In the movie Re;surrection in the video of the wedding, she can be seen curled up on his lap.
 * In non-Suzaku examples of intelligence, she is often seen with Cecile and looking at the screens like she may understand. And, when Emperor Lelouch thanks and dismisses his co-conspirators on the Avalon, after they all say "Yes, Your Majesty" she meows like she's joining in.