Gengutsu Hozuki was the Second Mizukage of Kirigakure and a member of the Hozuki clan during his lifetime
Gengetsu Hozuki was born into the Hozuki clan of Kirigakure many years before the start of the series and eventually became the Second Mizukage. During his lifetime, he had a mutual enmity with Mu, the Second Tsuchikage, and the two men eventually killed each other in what became their final confrontation. From this rivalry, the Second knew Mu's student, Onoki, and had picked on him repeatitively.
Second Mizukage Gengetsu Hozuki was revived by Kabuto Yakushi as a military weapon during the Fourth Shinobi World War, alongside his rival, Mu, the Third Raikage, and the Fourth Kazekage. After Mu was detected by Gaara through his sand, Kabuto had Mu summon the rest of the revived Kage and the Second immediately recognized his rival and the Third Raikage, but failed to recognize the Fourth Kazekage. When informed that he had been revived from the dead, the Second became confused until Mu reminded him that he had killed him many years before, at which time the Second reminded Mu that they killed each other, which Mu admitted to. As they pursued the Fourth Division, Gengetsu expressed disgust at having to fight with ninja from his own village, but was consoled by the Third Raikage, who told him of his certainty that they had been surpassed by the future generation. Upon seeing Gaara's enormous sand tsunami attack, the Second became excited and commented that the Fourth Kazekage had countered it effectively with his gold dust when he did so.
After Gaara arrived and began conversing with his father, Gengetsu reprimanded him for not realizing his son would have friends at his young age and, after hearing more, claimed that people without eyebrows are full of charisma. After being restrained by Gaara's sand, the Second, under Kabuto's control, was forced to counter with his Hydrification Technique, which allowed him to escape. Charging towards Gaara's Fourth Division, he and the Third Raikage then began to tell their powers and weaknesses as the Second summoned his Giant Clam. As the shinobi continuously failed to figure out his genjutsu technique, the Second grew irritated. After he had killed or critically hurt a large number of them, the Second embarrassingly admitted that he might be too strong for his own good when the division failed to beat him, even though he didn't want to win. When Gaara arrived, the Second noticed he looked winded and wondered if that was all he had when he used his Hydrification Technique to deconstruct Gaara's Shield of Sand. After Gaara managed to locate his Giant Clam and Onoki destroyed it, the Second attempted to kill the latter, but was surprised when it turned out to be a sand clone. After Gaara retrained him, the Second began to counter again, but was entombed in a pyramid of sand that was then bound with sealing tags.
All at once, a huge explosion occurred and the Gengetsu's smiling face appeared in the sky above. As most of the Fourth Division watched on in horror, Onoki informed them that this was the Second Mizukage's Infinite Explosions Ninjutsu, Steaming Danger Tyranny, and proceeded to explain the technique as a clone of the Second formed and landed on the ground. After the Second's real body, now rendered weak, was found by Gaara, the latter proceeded to trap him again in the sand pyramid as the clone continued to fight the rest of the division. Before he could be sealed, the clone ripped off the sealing tags on the Second's prison and he emerged from within it. Questioning the division as to how they would defeat him, the Second refused to help them further and told Gaara that if they could not defeat him with their power, then they couldn't expect to beat the enemy's leader, wondering if he was wrong to think Gaara was the golden egg of the five current Kage as his clone exploded again. Impressed when Gaara managed to protect everyone and himself, the Second told him he needed to stop the clone immediately. After Gaara disabled the clone using traces of his father's gold dust and the Second's hail contained in his sand, the Second beamed at the young Kazekage and declared that he really was a golden egg after all. As Naruto arrived and mistook the pyramid for the Second's body, the Second humurously said he and Gaara made a good duo, as the latter finally sealed him while he lamented that he hadn't had that much fun in a long time. He later vanished back into the afterlife when the Impure World Reincarnation was released. His soul, along with those of many other previous Kage, was summoned by the Sage of Six Paths, Hagoromo Otsutsuki, to help the undead Hokage teleport into and back from his mother, Kaguya's dimension with Team Seven and the dying Madara Uchiha. After Madara dies following a brief talk with his rival, Hashirama Senju, the Second and the other previous Kage were sent back into the afterlife by Hagoromo.
The Second Mizukage was typically very carefree and laid-back, but was shown to be easily excited and irritated. He had a mutual deep hatred for Mu, the Second Tsuchikage, which he valued more than his life, shown when the two killed each in their last confrontation. The Second took a great liking to Gaara, calling him the golden egg of the five current Kage and refused to help him with his Steaming Danger Tyranny clone so that he could prove his worth.
As the Second Mizukage, Gengetsu Hozuki was an immensely powerful shinobi, capable of critically wounding and/or killing a major portion of the Fourth Division by himself easily, using techniques so confusing, none them could figure them out. The fact that he was able to kill his rival, also a Kage, albeit at the cost of his own life, stands a large testament to his abilities. His power was great enough to pressure Gaara, who himself was the strongest member of the present five Kage at the time, after his revival during the Fourth Shinobi World War, even though he was probably not at full power given he was an undead at the time. Despite this and losing to the latter overall, Gaara openly admitted Gengutsu's skills were the strongest of the four Kage revived by Kabuto during the war, despite his rival, Mu, being revived at full power and having mastery of Dust Release. Gengutsu was a Yin Release genjutsu user and could summon a giant clam capable of releasing a mirage-making mist to aid him in battle. The Second was well versed in the techniques of the Hozuki clan, capable of using the Hydrification Technique to transform into or utilize oily water capable of destroying Gaara's automatic Shield of Sand and Sand Binding Coffin techniques. In the anime, the Second could create a stream of bubbles that could strike with great force, repel groups of foes, and oil down Gaara's sand as well. He could also fire bullets of water powerful enough to smash through a sand clone easily. Thanks to his Hydrification Technique, the Second was immune to blade weapons, like swords, which would cut through him, but he could simply splice his body back together.
Gengetsu's most powerful technique employed him using the oily water he could transform into to create a chibi-like clone of himself comprised of an outer layer of oil filled with water. When the clone exerted itself to take action, the outer layer of oil would heat up from its fast movements, causing the water inside to evaporate and make the clone continuously expand until it would violently explode. The water vapor from the explosion would then be blown high into the sky, where it would cool down and create hail that, in turn, would cool down and reform the clone, allowing the process to start all over. It was known as the Second's Infinite Explosions Ninjutsu because this process would never end unless the clone was somehow immobilized and cooled down simultaneously. The downfall to using this very powerful and dangerous technique, however, was that it greatly weakened Gengutsu whenever he used it, which may have allowed Mu to kill him. Nevertheless, however, notably, this technique, Steaming Danger Tyranny, was the one technique in the Second's arsenal that even Mu failed to counter and was, in fact, killed by.