To gather energy from Mika Kayama
Her mission was to help him gather the energy belonging to a champion French doll maker Mika Kayama who was suffering from depression after having her prized French doll damaged by Usagi Tsukino's young brother Shingo Tsukino. Nephrite who was performing inside his hideout, intonation with the stars; noticed that Orpheus was playing his lyre filled with sorrow. This tune showed Nephrite, his next target: Mika Kayama.
Under the orders of Usagi, Shingo goes to Mika to apologize her, but just before he is given that opportunity, Nephrite (under the disguise of Masato Sanjoin) visits the Kayama residence to discuss with Mika's mother the art of doll making. It is also during his visit that Nephrite implanted the essence of his youma Jumeau inside of it via his evil crest as he was examining the current doll Mika Kayama was working on. This caused Mika to become obsessed with doll making and "thereby hastened the young girl's approach towards her maximum" peak energy level especially when Nephrite requests that Mika make ten replicas on the doll she is currently working on.
Once Mika made her first replica of her doll, she became fixated with completing the remaining replicas of the dolls day and night, treating her mother badly every time she told her stop and even refusing to go to school. Shingo thought that this behavior was due to the damage he caused to Mika's prized French doll but Usagi as well as Luna thought otherwise, with Luna even suggesting activity from the Dark Kingdom. Usagi refutes Luna's claim but together with Ami Mizuno, the trio go to the Kayama's private doll exhibition at the Juuban Art Museum to investigate.
Usagi also took her young brother Shingo to private doll exhibition so he could give to Mika a present he created to her as a sign of apology. But when the brother and sister say hello to Mika, they both end up saying hello to the youma Jumeau who emerges from the original doll Mika was working on and drains all of the young girl's energy. Shingo attempts to help Mika, "only to be knocked against a wall by the youma. Because Shingo is unconscious, Usagi takes the opportunity to transform into Sailor Moon and challenge the youma".
Unfortunately, Jumeau drew first blood by manipulating the original doll Mika was working on, to lunge at Sailor Moon and attack her with its front teeth. Sailor Moon manages to dodge it and retreats outside of the Juuban Art Museum with Jumeau in hot pursuit. As Jumeau lunges at Sailor Moon, Sailor Mercury and Sailor Mars comes to the aid of Sailor Moon. Sailor Mercury's attempt at subterfuge by clouding the area with her Bubble Spray attack proved to be of little benefit as Jumeau simultaneously attacked the trio. Jumeau sends two replicas of Mika Kayama's dolls against the trio to bite them with their razor-sharp teeth. These dolls were restored to their normal inanimate state when Sailor Mars places her Anti Evil Scrolls on the foreheads of the dolls. However, Jumeau was able to find Sailor Moon in the fog and began to strangle her with her detachable hands. Sailor Mars uses her Fire Soul attack to destroy Jumeau's arms, but she was able to immediately regenerate and hurl them as sharp projectiles at Sailor Mars and Sailor Mercury whilst using her detachable hands to strangle Sailor Moon.
Sailor Mercury through her visor detected a structural weakness in her right foot. Sailor Moon tries to attack, but Jumeau's detached hands are sill strangling her. Through the intervention of Tuxedo Mask, all of Jumeau's limbs reattached themselves to her body (including the ones strangling Sailor Moon), giving Sailor Moon to destroy Jumeau at her weak spot.
Both Mika and Shingo return from their state of unconsciousness with Shingo apologizing for his previous action and Mika making a Sailor Moon doll for him.
Jumeau appears in the Super Famicom video game Sailor Moon
In the video game Sailor Moon: Another Story, numerous Jumeaus could be fought in Azabu-Juuban during Chapter One of the game.