Serenity Wheeler, known as Shizuka Kawai in the Japanese version, is Joey Wheeler's younger sister. The two were separated at a very young age when their parents got divorced.

Serenity wears light blue denim 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 Japanese version of the final episode's end credits, she wears a white shirt with some pink linings around the edge of its collar, sleeves and bottom, and a orange short skirt. In the manga, she wears longer sleeves and jeans rather than the shorter clothes that she wears in the anime. She has auburn hair with long bangs framing the sides of her face and a cowlick of four small hairs that cover the middle part of her forehead. She has greenish brown eyes.

Serenity is a gentle-natured and compassionate girl who is rather shy, fragile, vulnerable, and emotional. 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. 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.

However, Serenity has also shown to have inner strength, such as when she took off her bandages to see for the first time, to save Joey by diving into the water. After an inward struggle of self blame, she was the key to defeating Nezbitt and she pulled through. Optimism seems to be one of her key features despite nearly going blind, being aware she may not get the operation. 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 Téa even takes her sight-seeing at one point.

Serenity captures 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. She is always amused at their antics and even looks relieved at one point where Tristan drags Duke away from her.

During Joey's Duels with Odion and the Big Five, she expresses concern and even a bit of frustration at Joey's antics (when Joey drops his starting hand and when he begins freaking out over Yugi's "obvious trap" respectively), commenting "sheesh", before switching to an encouraging "nice catch", which the gang reacts to with dismay. In the latter she comments, "I wish my big brother would take this more seriously". During her day out with Téa, after Téa responds that she is sort of running late for her meeting with Yami Yugi, she responds with concern "don't 'sort of' me". The 4Kids Entertainment version tones most of this instances down and even changes one of them in the Odion duel to her dismay being toward the gang not offering Joey support. For example, here Serenity was the one who convinced Mai to prank the gang, while in the original version it was Mai who asked her to play along, with Serenity sticking with tongue out with a laugh and telling Joey she is sorry, something she also does when curiously asking Ishizu to use the Millennium Necklace to find out if Joey will face Yami Yugi or Yami Marik in the final rounds, to which Joey reacts negatively, prompting her to giggle. During the Tag-Team Duel against Nezbitt, she calls Nezbitt "metal-mouth" in the 4Kids