He was the host of the Sanbi, the giant three-tailed turtle. He was one of the only Jinchuuriki in history that could fully control his Bijuu, along with Killer Bee, which gave him immense power. Despite his youthful appearance, Yagura was the 4th Mizukage since at least the middle-aged Ao was a child.
It was during his reign where the Hidden Mist village got the nickname "The Bloody Mist Village." This was because the academy graduation exam pitted classmates against each other to the death, with only the winners graduating, although the practice was eventually discontinued after the young Zabuza killed the entire graduating class one year. His rule was also marked by internal conflict, and people with kekkei genkais were persecuted out of fear, such as Haku. It's also believed that Akatsuki originated in the Mist village under his reign.
Over a decade after the brutal graduating exam was discontinued, Zabuza would attempt a rebellion against Yagura, but failed and fled the village, doing missions as a missing-nin trying to raise money and forces to try again. Ao eventually gained a Byakugan from a defeated Hyuuga which he in turn used to uncover that Yagura was being controlled by an unseen force, something the villagers long feared. He would eventually die and Mei Terumi would succeed him as Mizukage while the Sanbi would run wild after his death until it was captured by Deidara and his partner Tobi for Akatsuki. Kisame before his defection discovered that Yagura was indeed being controlled when he was invited to join Akatsuki by Yagura's manipulator.
Yagura wouldn't be able to rest in peace long, as Kabuto used Impure World Resurrection to bring Yagura back from the dead as Kabuto's undead servant to fight in the 4th ninja world war, albeit without his Bijuu. He would later be modified by Madara into one of his own six paths of pain along with the 5 other deceased previous Jinchuurikis: Yugito, Roushi, Han, Utakata, and Fuu.
The group would run into Naruto and Bee where a fight ensues. The Jinchuuriki dominate the battle, until Bee goes full Hachibi and both destroys the surrounding terrain and sends the Jinchuuriki flying away with it's tails. To meet this new challenge, the Jinchuuriki use their V2 forms, using the full power of their Bijuus without fully transforming. They continue to overwhelm Naruto and Bee, until Kakashi and Gai appear to fight them as well.
Yagura favored a weapon that he carried on his back, an unbalanced staff with a small hook on one end and a larger hook on the other end.
Yagura has shown two ninjutsus, one is "Water Mirror" where he creates a mirror which spawns clones of whoever it reflects. The other is "Coral Fist" which causes the rock-like organisms to grow around the opponent after making contact with them and also fuses to whatever surface the person is touching, restraining their movements.
Yagura has full control over the Sanbi inside of him which means that Yagura could freely use the full abilities of a Jinchuuriki unrestrained. Based on Naruto or Killer Bee, this would mean that his Bijuu enhanced his healing and gave him a massive supply of chakra. He could also draw out the Sanbi's chakra in different stages like Naruto and Killer Bee, and could use partial or full transformations. As he drew upon the Sanbi's chakra, he'd get stronger and faster. The more he drew upon the more the demon's chakra would surround himself in the form of the Sanbi, getting more dense and protective while simultaneously sprouting more tails as he became stronger. Once he sprouts three tails, he'd be in his full Bijuu form and would be able to use the incredibly large and destructive Bijuu Bomb. But he could sprout all of the Sanbi's tails without fully transforming into it, giving him the most power he can achieve in his human-size without becoming the monster itself.