Saki Mitsurugi, real name Grigio, is a mysterious woman who opposes the latest heirs of Rosso and Blu's Ultra powers, Katsumi and Isami Minato and the secondary antagonist of Ultraman R/B.
Having lost her brothers Blu and Rosso in the fight against Reugosite and in some extent, Ultras' war against Ultraman Tregear, Grigio's heart became embittered and determined to somehow restore her brothers and to take revenge against Reugosite, no matter the cost, justifying that the sacrifice of entire life on Earth being a necessary one. She also resented her late brothers' human lookalike and successor Katsumi and Isami, seeing them as fake Ultramen and amateurs compared to their predecessors, yet envious at the same time since they proved as capable as the former.
As much as she is fueled by rage and sorrow like Ultraman Belial in fulfilling her goals, what prevents Grigio from stepping in as low as the former is a redeeming quality in form of her love for late brothers. The sight of Katsumi and Isami protecting her from Reugosite's assault made her realized that part of Rosso and Blu truly live on in their successors as much as their Ultra powers. That, coupled from reflecting on her errors in her last moments, allowed Grigio to feel remorse from her actions and decided to pass her Gyro to her successor Asahi, all while stating that she is happy to have met her and the rest of Minatos. In Geed and Minato's fights against Tregear, Grigio passed her Ultra powers to Asahi as Rosso and Blu did to Kasumi and Isami did.
Saki Mitsurugi was originally Grigio, the younger sister of Ultras Rosso and Blu. As with Kurenai Gai aka. Orb before them, they begin their early lives as normal human-like aliens born on the war-torn Planet Sanja. Originally working as mercenary employed by Bakubarba, their harsh life resulted in the siblings constantly having conflicts against those whom they had wronged. Eventually, they discovered a better way to live upon arriving at Planet Alhara, where they assisted its inhabitants in improving their farms, though they had to move on due to their status as outlaws. They set out for Planet Alcatraz, from which Grigio learned about rumors regarding the peak of Planet O-50, the home of Gai Kurenai's mentor the Voice of Light, who could bestow those who were worthy of the power to become Ultras. Blu and Grigio however dismissed these as mere fairy tales.
While escaping from the Balsas Gang, the siblings crash-landed at Planet O-50, near the Warrior's Peak, which Grigio believed was fate. They soon met Frau, who offered to help them reach Warrior's Peak, only to be caught in the fight against the same gang who was after their heads. Unfortunately, the fight awakened a Margodon from its slumber, and in their dilemma, the Voice of Light appeared and presented them with the Gyros and R/B Crystals, which they used to defeat Margodon and the Balsas Gang, and to rescue Frau.
Like Gai Kurenai before them, the siblings set out for more adventures under the guidance of the Voice of Light and soon became recognized by the Interstellar Alliance (sans Grigio due to her being forced to harness the power of a kaiju instead of becoming an Ultra). Upon returning to Planet Sanja as part of their mission however, they see their former home had changed into a child-trafficking hub. Bakubarba tried to guilt-trip the siblings, only to be killed by Grigio in his rants. The Balsas gang heard of this and unleashed the monster Fearmonger to kill the siblings. The brothers tried to fuse the crystals of light and darkness into a new power, but were unsuccessful due to being unable to connect with Grigio's wishes. Luckily, Rosso, Blu and Grigio were still able to defeat Fearmonger without having to make any sacrifices.
The three siblings soon unknowingly caught in the war between Ultras of Land of Light and rogue Ultra Tregear. At that time, they were tasked by Voice of Light to fight Reugosite, a planet-eating monster that is actually Tregear's bioweapon created through corrupting genetic code of a Space White Blood Cell. The siblings managed to defend the Earth from it and managed to drive it away at the cost of Rosso and Blu's lives. Since then, Saki vowed to destroy Reugosite at any cost.
Saki Mitsurugi first appears after Ultraman Orb Dark defeats Horoboros to claim its R/B Crystal. She later summons Horoboros during a confrontation between Orb Dark and Ultramen Rosso and Blu and the monster defeats the Ultras.
Saki later offers her services to Makoto Aizen to assist him in defeating the Ultras. However, when she has the chance, Saki betrays him and sucks Cereza into an air vent, trapping him their and freeing Makoto from his possession. Saki then usurps control of Aizen Tech and begins using their resources to convert Earth into a bomb to destroy Reugosite when it returns.
Despite Mio Minato finding another way to destroy Reugosite by trapping the crystal containing it in another dimension, Saki insisted her way was the only way to definitively kill the monster and ironically ended up inadvertently causing its release. Upon being freed, Reugosite went on a rampage. Saki attempted to hold it off as Grigio Regina while AizenTech also tried to restrain the monster, but Reugosite managed to overpower Grigio Regina. Reugosite then attempted to finish Saki off with his Genesis Requiem, but his attacked was blocked by Ultraman Rosso and Blu. Reminded of the way her brothers sacrificed themselves for her in the past, Saki became Grigio again and sacrificed herself to defend the brothers from an attack by Reugosite.
Right before she died, Saki told Asahi her real name and passed onto her her Gyro before dissolving into light.