Luna, known as Ruka or Luca (see below) in the Japanese version, is Leo's fraternal twin sister and a Signer who possessed the front claw Mark of the Dragon. She and Leo aid Yusei Fudo, whom they admire and support, in his Duels. Luna is the younger, frail, calmer, more collected of the twins. Luna tends to get tired right after a Duel. She has the ability to communicate with Duel Monsters after having spent ample time with Duel Spirits in a different dimension during a coma. She and her brother also have a mini series before each episode in the Dark Signer arc known as Card of the week.
Luna lives together with her twin brother Leo in the Tops residential area. They spend most of their time alone together as their parents are usually at work. They were initially educated via the internet, so they spend most of their time inside. Later they went to Duel Academy.
Luna has been Dueling since the age of three. Once, she lapsed into a coma after a Duel, and did not awaken for a month.
During that month, her mind lived in the Duel Monsters Spirit World under the protection of Ancient Fairy Dragon, one of the Signer Dragons. Before waking up, she promised she would always be there to defend the Duel Spirits from evil that threatened the land. However, the pressure from undertaking such a task alone was too much and Luna had to forsake her promise, allowing the Duel Monsters Spirit World to fall later into the influence of Zeman the Ape King, who without much difficulty with the help of his army, turned most of the spirits into stone, and eventually imprisoned them. He was even able to capture and rock-seal Ancient Fairy Dragon's spirit into the mountains. Luna later returns and saves the spirits with some help from Leo.
Luna was selected as someone to participate in the Fortune Cup. She was told she was randomly selected, but was invited so Yliaster could confirm that she is a Signer. However, not up to participating, Leo planned to disguise himself as her and Duel in her place.
Due to their Marks, Luna was able to sense Yusei when he was shocked by the police officers. She and Leo rescued him after his Duel with Trudge and took him to their home. Yusei wanted to leave, feeling that he was putting them in danger, but Leo talked him into a Duel. After Yusei won, Luna and Leo manage to get him to stay for the night. They awake the next morning to find that Yusei had left after modifying their Duel Disks to fit their small arms.
The next day, Leo entered the Fortune Cup disguised as Luna, but using Zigzix's Signer-detecting technology,Lazar was not only able to determine that Greiger Dueled Leo instead of Luna, but also that Luna is a Signer.
Luna faced off against Professor Frank due to Rex Goodwin's consolation game, but in the middle of the Duel, both her and Leo are hypnotized. Luna's conscious self awakens in the World of Duel Monster Spirits, where she sees the degradation of the dimension and decides to uphold her promise from then on. Before she concludes the match, Luna is called a Signer by Frank, although unaware at that moment of its meaning or its relationship with her Claw Mark of the Dragon.
Only Yusei becomes aware of her being one, though he does not tell the others. Eventually, though, they learn this after his Duel against Akiza Izinski. She witnesses for the first time the appearance of the Crimson Dragonduring Yusei and Jack Atlas' title Duel. Because of the Duel, Luna, along with the 3 other Signers, are transported to an alternate dimension. There they are shown 2 visions; one of the Stairway to the Dragon Star, where the People of the Stars were worshiping the ancient Signers, and the other showing Satellite being destroyed, and in its place the appearance of a spider mark shines.
After that, Luna and Akiza are the only ones to witness Jack's defeat. As they all return to the stadium, she reunites with Yusei in order to leave through an underground tunnel. Luna talks to Yusei about the vision, who responds that he doesn't know what it means, but that in any case he must prevent it from happening. She also asks him what is a Signer, to which Yanagi intervenes in order to answer to her. Luna learns its meaning and its relationship with her along birthmark, along with the fact that that Yusei, Jack, and Akiza are Signers too.
After the Dark Signers started taking action, Luna has been having vivid dreams of the Five Dragons and theCrimson Dragon battling an unknown evil. Ancient Fairy Dragon gets pulled into an abyss, showing that the Dragon's card is currently with the Dark Signers.
As Yusei is about to come face to face with Kalin Kessler, his Mark of the Dragon reacts along with Luna, Jack, and Akiza's. She notices this and Leo does too as he is nearby. Luna is able to sense that Yusei is the reason of why her Mark of the Dragon glows. After Yusei takes a hit by "Earthbound Immortal - Ccapac Apu", Luna's Mark of the Dragon gets etched onto her arm.
Leo explains to Tanner and Yanagi that Luna is frail and therefore she can't go out much. Luna accompanies Leo, Tanner, and Yanagi to the Arcadia Movement, where they meet Sayer. He induces them to sleep with gas, wanting to make Luna a member of the organization due to her powers. Luna wakes up next to Akiza, where they both watch the Duel between Sayer and Leo. Akiza tells Luna that Sayer wants to test her brother to find out if he possesses any similar powers. Leo passes out upon losing as Luna calls out to him.
After the Duel, Akiza informs her that Leo isn't in danger. Luna clarifies Leo doesn't possess any powers, but he was still kept in a place with Luna. Akiza tells her that in any case the Arcadia Movement will be her new home, as she doesn't want Luna to walk through the same path she did; the path of being feared and abandoned by others because of her powers. Luna asks Ancient Fairy Dragon for help, but her spirit says that she cannot rescue her and that she is witnessing the posing threat of the Dark Signers. In order to set her free, Luna must join forces with her servant, Regulus, who will eventually appear to her. Once Ancient Fairy Dragon is set free, she promises to protect Luna from the enemy.
After Bolt Tanner frees her and Leo, while fleeing the building, she senses Akiza in a Duel as her Mark of the Dragon begins to glow. As she decides to find her, the four of them run into Misty and Akiza's Duel. As Luna notices the lizard Dark mark, she recalls the vision of the battle between the Crimson Dragon and the Earthbound Immortals again. Right as Misty is about to Summon her Earthbound Immortal, Luna's dragon birthmark protects her, Leo, Bolt, and Yanagi from being sacrificed to Summon Misty's Earthbound Immortal. Tanner gets her and Leo out in time as the building collapses.
After Akiza lands in the hospital in a comatose state, Luna is there along with her brother and Jack. As with the rest of the people there, she witnesses the Duel between Yusei and Akiza. After it concludes, Akiza decides to help them in their fight against the Dark Signers. After it, Luna along with Leo and the other Signers are invited by Rex Goodwin into his house, as he wants to reveal to them some secret concerning the Crimson Dragon.
She learns from him that those who are Signers exist within some sort of "fate", derived from the Crimson Dragon's power, one which cannot be escaped from. That strange fate was the one responsible for that all of the four Signers to have actually met in someway. Thus, the "Fate of the Signers" unites each and every one of Yusei, Jack, Akiza and her own destiny together in order to guide them all.
Nonetheless, according to Goodwin the four of them are not only Duelists chosen by the Crimson Dragon itself, but they are also the reincarnations of the ancient Signers themselves. Also already predicted thousands of years ago the four of them would encounter one another. She also learns about the Dark Signers' abilities involving their powers and use of the Earthbound Immortals. According to Goodwin, their powers that manifest in a Duel come from the underworld. Also as they appeared on the city, many casualties occurred just like in Satellite due to their Immortals. Luna along with the others realize the shocking truth behind the transformation of an individual into a Dark Signer: It can't be reverted due to the fact those individuals are dead and that they are resurrected because of a powerful phenomenon occurring on the other side of the Underworld.
After this she cheers Leo up, who feels he can't be a hero or protect her because of having lost to Sayer, by saying he doesn't have to be a hero, and to just protect her like he always has.
Like Jack and Akiza, she's present along with her brother during and after Yusei's match against Roman Goodwin. After it concludes, the rest of the Dark Signers show themselves, as their ancient battle is about to commence. Later on as each one of the Signers is set on defeating their respective opponent, they split up to each of the four control units of the Original Ener-D Reactor. As Luna and Leo are taken by Trudge to whereDevack is waiting, she instead travels into the Duel Monsters Spirit World in order to free Ancient Fairy Dragon. Due to this, Leo bravely challenges Devack to a Duel in her place, as he is determined to fulfill his promise of protecting her.
In the Spirit World, Luna enters the Minus World in search of Regulus. She gets chased down by an army of monkey spirits commanded by Zeman the Ape King, who is Devack's Dark Synchro Monster. The monkey army captures "Kuribon", "Sunny Pixie", and "Sunlight Unicorn". She evades them with the help of a spirit named Torunka. Luna eventually finds Regulus at Pojar Forest and tells to the Lion the promise long ago made to Ancient Fairy Dragon. However, because of the Cursed Needle in its ankle, he understands her in the opposite way. After a chase, Luna is finally able to dispose of the Needle and convince the White Lion of her righteous intentions by showing her Mark of the Dragon. After that Regulus, Luna, and Torunka set off toZeman's castle. They are able to get inside by tricking Zeman into thinking that Luna is a servant of Zeman and has caught Regulus. She also tells Zeman to curse both Regulus and Ancient Fairy Dragon in order to minus Ancient Fairy Dragon completely. Zeman falls for this and is about to release her.
However, Zeman finds out they were trying to trick him and orders his army to attack them. He is eventually killed by Regulus, and at the same moment, Leo defeats him in the real world. Devack then Summons his "Earthbound Immortal Cusillu" and attacks Leo, who withstands the assault. Torunka sends Luna back to the real world, where she helps her brother. Devack then attempts to use Ancient Fairy Dragon against Luna, but she forces him to sacrifice it in order to save his Earthbound Immortal from being destroyed, which allows her to Summon her Dragon to her side of the field. Once this is done, Regulus appears and breaks the Dragons' chains freeing it at last. Luna then proceeds to use both "Ancient Fairy Dragon", equipped with "Pain to Power", and "Power Tool Dragon", equipped with "Rocket Pilder", to destroy Cusillu and defeat Devack.
After the Duel, Luna picks up "Ancient Fairy Dragon's" card, and she also discovers a "Regulus" Monster Card, indicating that he will return in a future Duel.
Following Crow's unexpected Shadow Turbo Duel against Greiger, Luna along with her brother, Trudge, andCrow himself witness the rematch between Yusei and Kalin Kessler. After the extreme Duel between the 2 finally ends, Luna, Leo, and Trudge accompany Yusei as he decides to go to the Original Ener-D Reactor directly in order to Duel against Roman one more time.
She watches the Duel between Yusei and Roman as it ends up with Yusei´s victory. After the Duel, she watches in horror as Roman blows up the bridge and drops Yusei into the Ener-D Reactor. Like the rest she´s glad, when Yusei returns, though unconscious. After he finally wakes up and that everyone eventually goes up through the same stairs they used before, they all notice purple sparks in the distance. She then knows that not only has Jack won, but also activated the third Tower.
Knowing that the last Tower to be activated is Akiza´s task, Luna along with Leo, Trudge, and Yusei all go to where it is. Arriving at an old and abandoned carnival, they split and began looking for her and Mina, except for the twins who look for them together. After Sayer awakens Akiza's power and is transformed once again into the Black Rose, they notice the tornado that has materialized on the carnival caused by the Synchro Summoning of "Black Rose Dragon". As they get to the area where the Duel is taking place, they both notice Akiza´s powers having once again manifested even stronger. After Mina and Trudge arrive, they all watch the outcome of the match, with Akiza emerging victorious. However, they notice that its too late for the last Tower to be activated, as the sun has already set.
The Condor geoglyph appears in the sky, while the King of the Netherworld revives from the Original Ener-D Reactor's crater. Kuribon appears to Luna, telling her that they mustn't let the King reach the Condor geoglyph. As the King begins moving towards it, the Crimson Dragon appears and transports Luna and her companions to Rex Goodwin's mansion, where the Condor geoglyph is located. Goodwin stands at the top of the Stairway to the Dragon Star, where he reveals himself to be the final Dark Signer and attaches Roman's arm to himself.
Goodwin declares that a Shadow Turbo Duel will be held within the Condor geoglyph as a ritual to welcome the King of the Netherworld. As Yusei, Jack, and Crow accept the Duel in order to stop the resurrection of the Underworld Ruler, Luna and the rest of the party cheer for them. However, when "Earthbound Immortal Wiraqocha Rasca" is Summoned, it tries to cheat by Summoning a flock of demonic black birds to attack Yusei, Jack, and Crow. Akiza and Luna are able to provide aid by manifesting Black Rose Dragon and Ancient Fairy Dragon to fight the horde.