ARC-V Reactor, known as ARC-V (アーク・ファイブ Āku Faibu) in the Japanese version, is a device that appears within Duel Academy in the Yu-Gi-Oh! ARC-V anime and is the titular device of said anime. Leo refers to it as the "fifth dimension".
Leo Akaba constructed the machine for the purpose of uniting all Four Dimensions and reviving his daughter, Ray, by fusing her reincarnations back together (In the English Dub, it is also stated that the process will destroy the girls' physical bodies and erase their souls from existence.). In order for the process to work, Leo ordered his followers to card thousands of people in Xyz Dimension (and eventually extended to Standard Dimension) into cards to use their life energy to power up the machine. At this time, he tasked Dennis McField, Yuri, and Sora Perse to find and capture Ray's reincarnations, signified by the Four Dimension Bracelets that each of them possess. Yuri successfully captured Rin and Lulu Obsidian, but was unsuccessful with Zuzu Boyle as her bracelet prevented him from coming close to her, and his plan was further delayed after Celina escaped into Standard Dimension.
Later, after Celina was brought back by Barrett and brainwashed by a Parasite Monster, Leo used her to lead a troop to capture Zuzu. After disposing the Doktor and freeing the girls from the Parasite Monsters, Leo also transported the four girls inside glass tubes to the machine with four of them surrounding it. The operators then started the machine, gathering energy to sufficient levels, before sucking out the girls' life energy at 50%, destroying their physical bodies and souls, and finally transporting each of them into ARC-V to begin the Unification Procedure, starting with Zuzu at 60% , followed by Lulu at 70%, then Rin at 80%, and finally Celina at 90%, with the dimensions corresponding to each girls being absorbed at the same time. Once the reactor's power reached 100% the four dimensions will reunite and Ray will be restored to her original form. However, being impatient, Leo ordered the transfer of Lulu before the energy reaches the sufficient level at 70% and Celina's transfer before the energy reached sufficient level. This greatly impacted the outcome as ARC-V was activated at full power, Ray was not yet revived because of the girls' (apart from Zuzu) not being properly transferred , and the four dimensions slowly started to become one. However, the machine eventually recreated the Natural Energy Cards and revived Ray in her spirit form with the spirit of Zuzu residing inside her (In the English Dub, it's also subtly implied that Celina, Rin, and Lulu's spirits also resided with them due to the mixture of the girls' voice with Ray's at one point.).
Following the battle against Z-ARC and the Standard Dimension rebirth into the "Pendulum Dimension", the four dimensions were split again but remnants of ARC-V remained. This created Interdimensional Corridor Gates, connecting the dimensions with one another. Ray's fate was also unknown but Declan theorized that she most likely still resided inside the remnants of ARC-V. Leo managed to locate the remains of her spirit, trapped between the boundery of the four dimensions, though he was unsure that it was Ray and theorized that Ray's fragments would eventually revive as Zuzu Boyle while he theorized that the remains of Celina, Rin, and Lulu had perished.
Ultimately as Yuya Dueled several of his allies in order to restore Riley's smile as she was rendered as a catatonic infant, he faced Declan in a final Duel to test his qualification as a pro. Yuya succeeded in defeating Declan which in turn brought Riley's smile back and purifying Z-ARC, allowing Ray to be reborn as Zuzu after reacting to the Duel. Although Celina, Rin, and Lulu's physical bodies perished due to Leo's action, their spirits miraculously survived and are now residing within Zuzu's. 
- ↑ Yu-Gi-Oh! ARC-V episode 127: "Revival Zero"
- ↑ Yu-Gi-Oh! ARC-V episode 36: "Sora's Secrets: Part 2"
- ↑ Leo: But that's not all! When I send the four girls into the Reactor, their existences will be erased as they reunite into Ray!
- ↑ Leo: Young ladies, your life energy will further fuel my reactor!
- ↑ Operator: Reading confirmed! The reactor is approaching 60%. Transfer the subject Zuzu to begin the Unification Procedure!
- ↑ Leo: I don't care! Transfer Lulu now! Time is ticking. Just do it already!
- ↑ Operator: But Professor! We need more power before we can transfer Lulu into the Reactor!
- ↑ Ray (with Celina's, Rin's, and Lulu's voices): And end all this suffering! By restoring peace to the Dimensions and make everyone smile again!
- ↑ Yu-Gi-Oh! ARC-V episode 145: "Shay's Rage"
- ↑ Yu-Gi-Oh! ARC-V episode 148: "That's a Wrap!"