In Naruto, when Kabuto conjured the sixth Coffin to terrify Obito and demonstrate his might, it included Madara Uchiha. When it came to knowledge about a wide range of topics, Kabuto was an expert.
He was well-versed in a slew of topics that most people were blissfully ignorant of. It wasn't a secret to him what Obito's plan was. He was also aware that 'Tobi' was not Madara. Consequently, to join forces with him, he had to demonstrate to Tobi something that the latter was frightened of. This was accomplished simply by revealing to Tobi that he was aware of his secret.
Kabuto's 6th coffin in Naruto
Kabuto's conversation with Obito was one of the most revealing clues to Obito's identity. After a lengthy discussion over joining forces, Obito was about to turn down Kabuto's offer when the latter displayed a sixth body, shocking him to his core. It is fair to infer that Madara's body was in the coffin, which would have exposed Kabuto's knowledge of Obito's true identity and forced them into an agreement.
When Madara was brought back to life, he had Hashirama Senju's might at his disposal. The Five Kages were annihilated by Madara during the war, a demonstration of his colossal prowess. To make matters worse, Madara gained access to Rinnegan, which made him considerably more powerful than Hashirama.
To prepare for Madara Uchiha's return, Kabuto re-evolved his physique using the greatest traits from all stages of his life. The transplant of Hashirama Senju's DNA allowed him to access the First Hokage's skills.
The Reanimation Jutsu
Reanimation Jutsu is used to bring the dead back to life in Naruto. Orochimaru finished what Tobirama Senju started, but he never got around to it and classed it as kinjutsu. Kabuto, however, claims to be even better at it. To counter Kabuto's Reanimation Jutsu, Sasuke and Itachi joined forces.
Despite this, the brothers' Susanoo onslaught against Kabuto became more difficult due to his ability to harness Nature Energy. Kabuto saw Itachi as a formidable shinobi in Naruto. His solution was to revive him and use him as a savage ally. However, if Kabuto had not resurrected Itachi, the outcome would have been different.
Usage of the reanimation Jutsu
In the world of Naruto, to apply this method, the user needs to first get a sample of the person's DNA. While dried blood and tissues after the victim's death may also be used, Kabuto thinks this to be a grave-robbery. The reborn soul also needs a live sacrifice to serve as a vessel. If souls are easily accessible in the Impure World, they may be reborn by this method.
An Impure World Reincarnation scroll is used to smear the DNA of the person to be reincarnated. A living sacrifice is enshrined in this particular seal, which is triggered when a scroll is opened. The sacrifice gets covered in dust and ash and is molded into the appearance of the person to be reincarnated at the time of their death.
What if Rin was in the 6th Coffin
In Naruto, Edo Tensei stipulates that to bring Rin back to life, Kabuto must have her DNA. Obito may have taken Rin's corpse with him or buried it in the Konoha cemetery, but no one knows for sure. Obito may have buried her in a place where no one could discover her. If that's the scenario, Kabuto will not be able to get her DNA since she was not a well-known shinobi before her death, and Obito, as we've seen, is a master of disguise.