"Playtime is over, you know."
— Juubey, chapter 4.
Despite being an incubator, an emotionless race, Juubey has displayed more emotion throughout the manga than his colleague Kyubey has in the entire anime. He has displayed annoyance/embarrassment when he was violently hugged by Satomi, disgust when he was fed an evil nut instead of a grief seed and fear when he sees Yuuri’s sigil form beneath the Pleiades. He also expresses distaste in being called “Bey-chan” by Satomi.
Notably, Juubey appears to understand the concept of deception: when asked, Juubey says he doesn’t know why soul gems turn into grief seeds and claims that it only happens sometimes. He appears to outright lie to Kazumi and the Pleiades about Niko’s preliminary actually being Kanna Hijiri, the product of her wish, something Kyubey never does. He also appears to be sad when he tells Kazumi that returning a grief seed back into a soul gem is impossible. When describing the difference between Niko and Kazumi’s magic, he calls Kazumi’s magic “sacrilege” but quickly corrects himself saying “destruction” instead (Japanese version only).
While around the Pleiades, Juubey behaves akin to that of a usual magical girl mascot and provides information when necessary, though in some circumstances he has been shown looking down on them with an evil grin, especially when he explains the details of the Freezer to Kazumi.
Another trait that separates Juubey from Kyubey is that while the latter stays completely neutral when magical girls turn on each other, Juubey openly speaks out and offers his assistance, such as in chapter 8 with Kazumi and Niko on the best strategy to neutralize the Soujus and appears worried when they are clearly outmatched by them. Overall, his actions are contradictory to his function of being an incubator that harvests emotions.
Juubey in Kazumi Magica
Juubey first appears in chapter 4 after Kazumi and the Pleiades rescue Umika and Kaoru from a witch, watching their celebration from a rooftop seemingly alongside Kyubey. He jumps down and greets the girls, much to Kazumi’s surprise, and is introduced by Satomi as the fairy they all contracted with. Juubey then proceeds to absorb the grief inside the Pleiades' soul gems by spinning in the air until they are sparkling clean and then eats the grief seed they've collected from the witch. When he tries to dispose of evil nuts however, he spits them out in disgust and says they're not real grief seeds. Soon after, he and the Pleiades are attacked by Yuuri.
Juubey returns to Umika's house with the Pleiades after Yuuri's attack, and is seen sleeping with Kazumi's cat Toto as the girls discuss Yuuri's message. He accompanies them to the Asunaro Dome to meet with Yuuri, and at Saki's command, attempts to purify Yuuri's soul gem during the Pleiades' fight with her. However, Yuuri shoots Juubey as he begins spinning and becomes a witch. He gets up shortly afterwards, still injured, and explains to Kazumi what happened to Yuuri, claiming that he doesn't know why magical girls turn into witches, but that it happens "sometimes". He goes on to explain to Kazumi that it isn't possible to revert the process and that Yuuri must be killed.
Some time after the events of that night, Juubey accompanies Niko and Kazumi on a "witch hunt" to the La Vie Land Amusement Park. He remains with them during Niko's fight with Ayase Souju, and explains to Kazumi Niko's abilities and Ayase's transformation into Luca. When Kazumi asks him what happens if a magical girl's soul gem is lost, he answers that they lose their magic and some may die "depending on the situation". During the battle, he defines the Souju's "Picchi Gemellati" attack to Niko and identifies Kazumi's magic as "destruction". Finally, he helps Kazumi and Niko launch a synchronized counterattack against Ayase by biting their hands.
After Souju is defeated, Niko unexpectedly becomes a witch and is subsequently defeated by Mirai; however, another Niko appears, who Juubey identifies as Niko's "preliminary", created by her contract. Juubey is then seen accompanying the Pleiades to the Freezer, where he explains to Kazumi the real connection between a girl's soul gem and her body. Umika then restores Kazumi's memories with magic, triggering a flashback when the girls first met Kazumi and Juubey.
In the flashback, a Kyubey-esque silhouette appears alongside Kazumi as she saves the Pleiades Saints from a witch. Stepping forward, it is revealed to be Juubey, who states what he is the one who Kazumi contracted with. Kazumi explains that she met Juubey one day after being rescued from a witch by Mami Tomoe. He offered her the contract, which she used to make her grandmother wake up again rather than cure her illness so she can spend one more day with her. Oddly, Juubey is seen next to Mami as she speaks to Kazumi, and his personality and tone of voice closely resembles Kyubey's than his own.
Juubey then makes contracts with the six Pleiades Saints, and offers them advice as they fight witches for some time. One night, he hurriedly calls the Pleiades out to the Asunaro Dome, where they see Yuuri Asuka becoming a witch. They demand that he tells them what happened, and he reveals the secret behind the soul gems and grief seeds, and that his goal is harvesting the energy produced when magical girls despair and become witches. When Kaoru attempts to hurt him, Umika grabs him and suddenly headbutts him and fires lasers into his eyes. Juubey then falls the ground, speaking in a different tone of voice and expressing confusion as to why Umika is mad at him. Umika explains that she has altered Juubey's memories to prevent him from making anymore contracts and that he'll work for them now. The story then returns to the present.
Juubey is next seen on a rooftop with Niko, watching from afar as Satomi reveals to Kazumi that she is Michiru Kazusa's clone and her memories are fake. He remains with the Pleiades after Kazumi had fled, and is next seen with Niko again as the Pleiades receive telepathic instructions from Kazumi to meet her at the Freezer. At the Freezer, the Pleiades speak with Kazumi, and prepare to kill her at her request as she becomes a witch. After she dies, however, her body suddenly turns into a toy clone of Niko, and the actual Niko steps out with the real Kazumi safely in tow. However, Niko suddenly turns on the group, forcing Saki to become a witch with an evil nut and killing Mirai in the process. Niko reveals herself as Kanna Hijiri, and explains in a flashback that she learned from Juubey that she was created from Niko's wish for another version of herself who had no memory of her shooting her friends when she was a child. She then made a contract with Juubey, wishing "to connect" to the Pleiades and sneak up on them without being noticed so that she can get her revenge on Niko and the Pleiades for being artificial. She attacks them by using Mirai's magic to free the darkened soul gems from the Freezer so that they can complete their transformation into witches, this including the Souju's soul gems.
During the battle against Kanna, Kazumi's soul gem blackens as she begins reverting back into a witch and Juubey quickly attempts to purify her gem at Umika's order. However, Juubey stops mid-spin and falls to the ground breaking into pieces. He himself doesn't know what is happening to him and says "Ciao" to Kazumi one last time before vanishing completely and leaves behind a pair of broken grief seeds. It is then that Kyubey appears, watching from afar, commenting that "your 'Juubey' was incomplete".
As Umika's gem is also beginning to break open, her magic begins to fail. A huge magic circle of hers, floating over the city, shatters, and Kyubey appears just as she and Kaoru shout for him to reveal himself. Kyubey appears, saying it's been awhile and introduces himself to Kazumi for the first time.
It is revealed that, as the Pleiades were creating the Freezer and planning to revive Michiru, Niko created an artificial incubator with her magic using a body of Kyubey that was killed by Umika. Having studied the mechanics of grief seeds, she incorporated their purification capabilities into the incubator corpse, as well as making it obey the Pleiades. She dubbed it a "Dependent-Version Incubator", and nicknamed it "Juubey". Umika then cast a wide-scale spell over all of Asunaro City as part of their plan, making it impossible for anyone within the city to perceive Kyubey at all, as well as editing their existing memories to remove Kyubey and replace him with Juubey instead, suppressing their memories of how to purify soul gems without Juubey.
Like Kyubey, Juubey also possesses some of the same abilities that all incubators have. Juubey mainly uses them to assist the Pleiades Saints.
- Grief seed consumption: As Kyubey consumes used grief seeds through the hatch on his back, Juubey does the same through the circle on his forehead. While Kyubey’s acts like a simple receptacle, Juubey’s is more akin to an actual mouth as it can be seen clenching just like one’s mouth does while eating and munching sounds can be heard.
- Soul gem purification: Unlike Kyubey, Juubey has the ability to clean and remove the grief from soul gems, allowing the girls relief from having to compete with others for grief seeds. In chapter 21, it was revealed that Juubey was imperfect in this ability, as it is implied that cleaning the gems would only be effective if he had consumed a grief seed beforehand. Instead, he was only able to polish the surface of the gem so it appeared to be clean while the grief accumulated inside.
- Telepathic communication, invisibility, and teleportation: It is assumed that Juubey had these abilities as well due to being an Incubator. It was thought that he could also grant wishes, create soul gems, and replicate, but chapter 21 reveals that Umika’s magic made him appear to have these abilities when it was actually Kyubey performing them. It is also notable that whenever Juubey was injured, he never showed any replication abilities, just recovery (possibly due to the Pleiades’ magic keeping him going).
- It appears that he absorbs the grief from the soul gems onto his skin, which is appropriately pitch black. On the other hand, he is seen making a gulping noise and licking his lips afterwards, suggesting some sort of consumption.
- The original series proposal for Madoka Magica referred to the contract-making creature as a fairy. Juubey being called a fairy may be a reference to that.
- Whenever Juubey cleans the girls' soul gems, he spins quickly in the air, resembling a black hole.
In chapter 21, it is revealed that Juubey’s name (Jūbē) is short for "Dependent Incubator" (Jūzoku suru inkyubētā) — an incubator under their jurisdiction.