Runa Shijo is an 18 year-old human girl, the younger sister of Ren Shijo and Aki Kiyo and the daughter of Kuso Kiyo and Ayden Shijo. She duels with a Spell Counter Deck and her Duel Spirit is Dark Leopard, and she is the love interest of Jaden Yuki. Runa has been on the run from a mysterious force for two years when she is forced into a duel by someone called, Mr. T.
Runa is a slim girl of average height and weight with dark brown hair with streaks of golden-orange and bright red eyes, which she inherited from her father. She wears a long-sleeve white and blue zipper hoodie, a sleeveless dark turtleneck sweater, a dark blue skirt with black leggings, white knee-high boots, and black fingerless gloves. Her red Deck Case is strapped her upper left arm and wears a KaibaCorp Duel Disk on her right arm.
Runa is a very open and friendly person, but does have a very short temper. She has a strong relationship with her older brother Ren, who raised her since her mother passed away. Runa doesn't take it when people make fun of weak cards.
- Duel spirit communication: Like her mother, Ayden, and brother, Ren, Runa is able to see and communicate with all Duel Spirits. In addition, Runa is able to sense a Duel Spirits feelings.
- Connection to Spirit World: Ever since she was young, Runa has a strong connection to the Duel Monster Spirit World. She is able to go to and from but has only been able to go when she is asleep.
- Dueling proficiency: Runa is a very skilled duelist.
- Multilingual: Runa is able to speak and write fluently in Japanese and many languages.
- Karate expert: At a young age, Runa started to take karate lessons and soon rose in ranks and currently has a third-degree black belt and is working on her fourth-degree.
- Agility: Through her karate lessons, Runa has become very agile, able to move quickly without being seen.
- Strategy expert: Runa is very skilled in dueling able to adapt and/or improvise with whatever she has on hand
- Hand-to-hand combat: Taught by Ren, Runa is skilled in physical combat and can easily defend herself.
- Athleticism: Runa is highly athletic and very physically fit.
- Cooking: Runa is an adept cook and is able to make dishes from many countries.
Runa is the youngest daughter of Kuso Kiyo and Ayden Shijo and the younger sister of Aki Kiyo and Ren Shijo. However, her mother passed away shortly after her birth, leading her older brother to care for her.
When she began to duel, Runa found out that she could talk to them. Scared, she went to her brother and asked her. Ren then told her that she inherited her ability to see Duel Spirits, just he had. Ren then gave Runa Ayden’s Deck and prized card, Dark Valkyria. Around the time Ren was 18 and Runa was only 8, Ren had enough of their father not caring for Runa and more focused on training Aki, so he took her and their belongs and ran away. After running away, Ren had his and Runa's surnames changed to Ayden's. They have not spoken to Kuso or Aki since then.
Around the time Runa was 13 years old, Ren signed her up to attend a Dueling School. Unfortunately there, Runa was bullied for 'talking to thin air'. But one day, a young boy stopped the others from bullying Runa. That boy was known other than Kaito Hattori, a Ninja-Dragon-Type Duelist. The two formed a fast friendship becoming best friends.
Once she turned 16, Runa asked Ren is she could travel around the world, to which he agreed. However, she was 18; Runa was forced on the run. She doesn't know who, but she can feel that someone wants her out of the way.
- Runa's Duel Disk is a combination of the 5D's Duel Disk and the GX Duel Disk