We learn that Utakata is a rogue ninja from the Hidden Mist Village with a massive bounty of 50 million ryo on his head.
Naruto realizes that Mizuki’s transformation looks eerily similar to Orochimaru’s Cursed Seal , revealing that Mizuki was a disposable pawn for Orochimaru all along. Naruto Shippuden : Episode 146
"The Successor's Wish" Arc: Six-Tails Unleashed (Filler/Mixed Canon)
Just as the bandits corner her in the woods, Utakata (the Six-Tails Jinchuriki) arrives to save her. He uses his unique bubble-based ninjutsu to fight off the attackers and escape with Hotaru.
Hotaru returns to her village thinking she is safe, but her own uncle betrays her to a group of bandits who want the forbidden jutsu.
Mizuki breaks into a Nara clan research lab to steal a special chemical formula. By the end of the episode, he drinks this potion and transforms into a massive, tiger-like beast.
If you want to see a fan reaction and breakdown of the events in the Shippuden episode, check out this review: