Mystie is an alien who came to Earth many years ago as a young boy, who took the name Michiro (未知郎 Michirō). He drove away a group of bullies who were targeting Sachiko Motomiya, and helped her rebuild her sand castle afterward before departing again. He was hired by Foreign Affairs Division X as an undercover agent in the Villain Guild. Mystie protects Sachiko from the hitman Alien Bado El-Ray, who was sent to assassinate Sachiko before she could complete her new device which could discern aliens from humans with a simple scan.
Just as El-Ray was about to shoot Sachiko, Mystie jumped at her and pushed her out of the way of the bullet, before bringing her to safety. Sachiko recognized and remembered him after taking a quick glance at the marking on his hand, but they had no time to talk as Mystie quickly left the scene.
Mystie had to meet up with two "accomplices" of his, in replacement of an Alien Serpent who was supposed to arrive instead. He had planned to stop the both of them before they could call Aribunta from the ground again, but his plan was ruined by Homare Soya, who was nearby and wanted to capture him after eavesdropping on their conversation. Despite that, they worked together to defeat the two alien henchmen with Inspector Sakura's help, but they did manage to summon Aribunta, who was dealt with by Ultraman Titas and then Ultraman Taiga Tri-Strium. Homare and Mystie were backed up by Inspector Sakura. After the battle was over, Mystie said goodbye to Sachiko and left to America, reminding her of their childhood encounter once more.