Dark Yuki is a physical manifestation of Yuki Jojima's darker emotions as well as the eleventh Horoscope, the Gemini Zodiarts, who had evolved past the Volpecula Zodiarts form.
Identical to Yuki to every way, the only physical difference between the two is that Dark Yuki normally wore a white expressionless mask on her face. However, when she starts to become the dominant aspect, Dark Yuki is able to discard the mask until Yuki regains herself.
As the embodiment of Yuki's inner darkness, Dark Yuki is sadistic and calculating and acts on her other self's evil desires. She also has a desire to replace the real Yuki in order to become more real.
In her human form, as she becomes the dominant Yuki, Dark Yuki siphoned the original's traits and knowledge. As the Gemini Zodiarts, not only does she possess a very unorthodox fighting style, she is armed with several playing cards that function as explosives: red cards known as Lynceus that explode on contact and blue cards called Idas that are remote explosives that explode when she snaps her fingers.
Her Supernova ability allows the Gemini Zodiarts to produce a clone of herself that doubles as a living explosive.
Dark Yuki came into being when the Leo and Libra Zodiarts forced Yuki to activate a Zodiarts Switch. Due to the nature of evolution concerning a Gemini Zodiarts, only the dark aspects of her personality congregated into the newly created Dark Yuki.
Dark Yuki would then go on to do the things Yuki could only dream of: cheating Kengo out of a chance to see NASA, vandalize school property with space themed graffiti, and pushing Miu to a five foot drop. The Kamen Rider Club thought Yuki snapped until they found footage of Dark Yuki. The matter gets worse when it is revealed that Dark Yuki is becoming more real every minute while the real Yuki is slowly being erased from existence, taking over Yuki's life and tricking the Kamen Rider Club into attacking their now-masked friend.
With the original Yuki becoming less and less dominant, the Horoscopes began the ceremony of death and rebirth where Yuki would disappear forever. However, when Gentaro shows the ticket Yuki gave him to go to space with her when Gentaro moved away back when he was a kid, Yuki regained her face and Dark Yuki lost dominance.
Enraged, Dark Yuki became the Gemini Zodiarts, but the transformation did nothing to sway the weakening. While initially having an advantage over Fourze in Cosmic States, the Rider used the Meteor Storm Switch after Ryusei threw it to him to absorb the Cosmic Energy of the Gemini's Supernova technique and one of her red cards, and later destroyed Dark Yuki with the Rider Super Galaxy Finish.
Her Horoscopes Switch was retrieved by Leo.
An alternate romanization of Yuki Joujima's name is "Yūki Jōsima", which is an anagram of "I am Sōjikyū". Sōjikyū is the term for "Gemini" in Japanese astrology.
- Gemini's motif is based on the Gemini constellation, a jester and a harlequin.
- The harlequin is an ancient type of clown that is said to have inspired the Helliquin, a demonic black face, clown servant of the devil in certain plays. Which makes sense giving Dark Yuki/Gemini's dark nature.
- The Gemini Zodiarts is a very subtle call to Kamen Rider Double: the outfit is two-toned in color, with black making up the left side, and the motif of Gemini being a jester and a harlequin.
- The Zodiarts suit is also somewhat similar to the design of the Nasca Dopant suit.