Serenity Wheeler, known as Shizuka Kawai in the manga and Japanese anime and Shizuka Jonouchi in the first series anime, is Joey Wheeler's younger sister. The two were separated at a very young age when their parents got divorced. In both series of the anime and the manga, Serenity is depicted as being a sweet natured, gentle, fragile and dependent young girl. She has a small fragile body, flowing auburn (brown/red) hair, and gentle dull brown eyes. Throughout the whole series she is pictured as gentle but strong, with a never changing attitude of high spirits.
Serenity's character design was overseen by Kazuki Takahashi. In the second series anime, Serenity wears light blue shorts and long white socks with Blue sneakers. Her top is a sleeveless pink and white shirt with a high collar that is worn on top of a short-sleeved yellow shirt. She does not change this outfit throughout the series except when she is in the hospital, where she wears simple pink pajamas. The photo Joey Wheeler has of her in his wallet shows her dressed in a sailor fuku common to middle school students in Japan. In the manga, she wears longer sleeves and jeans rather than the shorter clothes that she wears in the anime.
Serenity is a gentle-natured and compassionate girl who is rather dependent on her friends and seems to be generally more timid than the other females. She is also shown to be extremely trusting to the point of naivety, believing Tristan when he says, in an attempt to impress her during her recovery from her operation, that he taught Joey everything he knows about Duel Monsters. However Serenity has shown to have her own inner strength, such as when she took off her bandages to see for the first time, to save Joey by diving into the water. Serenity also after an inward struggle of self blame, was the key to defeating Nezbitt and she pulled through. Optimism seems to be one of her key features, which one would need to survive nearly going blind, being aware she may not get the operation. (Prior to Duelist Kindgom.) Serenity was separated from her older brother Joey to live with their mother when their parents divorced, but they seemed to have kept in contact over the years, as Serenity looks up to him and they both love each other dearly; during Duelist Kingdom Joey is seen to have a picture of her in his wallet. Joey often states that Serenity was his biggest inspiration throughout Duelist Kingdom and Battle City. She shares a strong bond with Téa Gardner and Mai Valentine, and Tea even takes her sight-seeing at one point.
Serenity's character is portrayed as a very pretty young girl, capturing the romantic interests of both Tristan and Duke. This leads them to constantly argue over her during the Battle City arc and both eventually end up giving her their number. It is never shown if she ever liked either of them as more than friends, though she is always amused at their antics and even looks relieved at one point where Tristan drags Duke away from her. But she did seem to be more fond of Tristan than of Duke. Her innocent nature was even able to make one as self-centered as Seto Kaiba be considerate enough to get medical help for Bakura even during the Battle City Tournament.
In the first series anime, Shizuka was ill. She was visited by her brother Joey and her brother's friend, Yugi Muto, while she was in the hospital. Joey gave her a present. Later, Tristan Taylor, Téa Gardner and Miho Nosaka, alongside Yugi and Joey visited her again. Dr. Kekeru Goyu was supposed to be caring for her, but was slacking off, causing Nurse Miyuki Sakurai to get angry at him. After, Dr. Goyu fires Miyuki for hitting him, but Yami Yugi defeats Dr. Goyu in a Shadow Game.
In the manga and second series anime, Joey receives a video tape from Serenity. In it, she tells him that time is running out for her because she needs an operation to repair her eyesight or else she will go blind. The operation, however, is very expensive and to pay for it, Joey needs to win the three million dollar prize at the Duelist Kingdom tournament. Despite not having an invitation, Joey is allowed on the ship after Yugi gives him one of his Star Chips. Yugi also gives him the three million dollars he won to Joey so he can pay for Serenity's eye operation.
After her operation, Battle City begins. Serenity spends most of the tournament in the hospital recovering with Tristan taking care of her. She finally removes the bandages around her eyes and sees clearly for the first time when the entire gang is sent to the Domino Pier, where Yugi and a Marik Ishtar-controlled Joey are Dueling. After Joey breaks free from Marik, he and Yugi are dragged into the ocean. Joey sets Yugi free from his shackles and Serenity dives in with the key for Joey's chains and saves him. In the manga Shizuka is present, but Joey Wheeler is instead rescued by Kaiba, who simply drops the key needed into the water. Afterwards, she hangs out with the gang for the finals and becomes friends with Mai Valentine and Téa Gardner, who both treat her like a younger sister.
During the Virtual World arc, she Duels once alongside Tristan Taylor and Duke Devlin against a member of the Big Five, Nezbitt but is not very experienced. She goes into a state of shock after Tristan loses, feeling responsible for getting him involved in the first place, but regains her confidence and, reflecting on the basic rules she learned from Tristan earlier, barely wins.
Serenity was in charge of taking care of Mai Valentine, who was unconscious after losing to Yami Marik. During Joey's Duel she was asked by Ishizu Ishtar to go and see the Duel while she took care of Mai, but when she arrived, Joey fainted and lost the Duel. When the tournament ended, she went back to Japan.
One day, Téa was walking through Domino City and met with Serenity. Téa showed the city to Serenity, she explained where some Duels took place and other important events.
Serenity is seen briefly in the Waking the Dragons arc, when Joey calls her to say goodbye, as well as a montage narrated by Maximillion Pegasus just before the Ceremonial Battle, as Pegasus says that everyone whom Yugi and his friends met was destined to play a role in his journey. She is not seen again until the last episode during the Japanese version's end credits, though in the Japanese version, she is mentioned by Tristan saying greetings to her in the Grand Championship arc when he realizes he is on live TV.
Her Deck is comprised of EARTH and LIGHT Female monsters. In the second series anime, Serenity only Dueled once, in a three on one Duel with Tristan and Duke against Nezbitt during the Virtual Realm arc. She does not Duel in the manga or the first series anime. Most of the cards in her Deck are from Labyrinth of Nightmare and Legacy of Darkness.