Miyu's main source of power throughout both series is her Mythril Dress, which changes color depending on the weapon being used. She is apparently extremely fast, as she is able to cut a moving missile fired by Natsuki's CHILD Duran with little effort. The My-Otome version of this dress replaces the skirt with short-like pants. She houses an unknown amount of internal weaponry inside her body, including her "Maria" sword arm (Navy Sword Mode), her default mode; a chainsaw (Ochre Mode), a gatling gun/artillery (Scarlet Mode), a drill (Khai Green), and an anti-materializing weapon (Platinum Secret). Also, in Otome, her arsenal is redesigned: Instead of a gatling gun, she possesses triple machine gun, and in addition, her "Maria" sword arm can upgrade to the gold sword arm, "Artemis" with the help of her pet bird, Alyssa. Note that this upgrade is accessed through equipping the "Millennium Queen", a reference to Mai-HiME's Alyssa.
Miyu initially appears as a girl in Mai Tokiha's class and the adopted daughter of Joseph Greer, the resident priest of Fuuka Academy. She is friendly towards Mai, making strange comments about the exact temperature and amount of precipitation, and is very protective of Alyssa Searrs.
After encountering Akane Higurashi, whom she saw fighting an Orphan, Miyu is revealed to be an android created by Joseph Greer of the SEARRS organization to protect Alyssa, as being a "fake" HiME Alyssa is unable to materialize an element and hence relies on Miyu for protection. Her name is an acronym for Multiple Intelligencial Yggdrasil Unit. She lures Akane out with an Orphan and forces her to summon her Child, Hari, and upon defeating the Orphan Miyu kills Hari with her anti-materializing weapon, and by extension Akane's boyfriend Kazuya, demonstrating the dire consequences of losing in the HiME Carnival. She is later seen observing the HiME via a surveillance room in the basement of the Church and looking after Alyssa until Natsuki Kuga arrives and confronts them only to be knocked out, though Miyu advises Greer not to kill her because it "is unwise to kill a Valkyrie without her Child present".
When the true intentions of the SEARRS organization - to replace the real HiMEs with Alyssa and hence gain control of the power of the Obsidian Lord - become clear Miyu takes an active role against the HiME, though with the combined efforts of the HiME and Alyssa's Child's defeat she flees with Alyssa in the hope of finding safety. However, Joseph Greer, who had earlier been dismissed for incompetence after finding that the codes he had installed in Miyu had all been erased, appears and shoots Alyssa before himself being dispatched by Miyu in retaliation. As Alyssa dies Miyu is visibly distraught and cries that the "Golden Light" she loved has faded. She carries Alyssa's body into a frozen lake, where it is assumed Miyu herself perished.
In this incarnation Alyssa is actually Natsuki's sister, though Miyu still serves as her protector. She fights Nao's Child and defeats it, forcing Nao to change her Key to Takeda.
Miyu also appears in My-Otome as a friend of Arika Yumemiya. She appears to Shizuru in episode 14 and reveals knowledge which suggests that she might be the same Miyu from My-HiME, however, her name is revealed as the Merciful Intelligential Yggdrasil Unit, instead of the Multiple Intelligential Yggdrasil Unit that she was in My-HiME. She may have changed her name to reflect her change in status from an assault unit to a protector. She travels with a small bird named Alyssa (who shares her HiME counterpart's hairstyle). Whether this is the same Miyu as in My-HiME is the subject of much debate among fans, but no conclusive evidence has been revealed so far.
She was apparently created by My-Otome's version of Alyssa Sears in the past in order to deal with the presence of the Ultimate Black Diamond that Nina Wáng uses. She also appeared during the invasion of Windbloom years before, but came too late to stop it. There is indication that she was very good friends with the former Blue Sky Sapphire Rena Sayers, as a vow she made to Rena that she would destroy Schwarz and those associated with them is mentioned. In addition, she rescued Arika as a baby and gave her to her grandmother (Rena's mother) and also found Rena's GEM in Artai and took back to be inherited by Arika when she got older. It also appears that she watches over Alyssa's descendants, including Arika (whose hair she sees as glowing like Alyssa's when she used her HiME powers), and divulges important information about Arika's origins and the Harmonium.
She has her trademark "Maria" sword arm and when she teams up with Alyssa, it activates her attack, "Artemis", an energy sword slash. She later pulls out her Mithril Dress from a lava pit. However, her weapons are modified compared to her My-HiME version. In the finale, she goes to what looks like Fuuka Gakuen and actives the Guiding Star which causes all the Otomes to materialise without their masters.
In the alternate take on the My-Otome universe present in the manga Miyu is a MAID, a super intelligent android as part of a series as the basis for the others, she was excavated and repaired to working condition by Alyssa Kruger. She first appears in Chapter 14. In this chapter and 38-40 it is heavily implied Miyu is the same Miyu as the one from the My-HiME manga. She demonstrates her ability by catching a large number of dishes, then destroying a machine gone amok with her trademark efficiency. In a competition to test the ability of Otomes against hers, she thoroughly trounces the challengers. She retains the Mythril Dress of her other incarnations and first uses after the team of Arika, Nina and Erstin apparently defeat her.
When Alyssa Searrs from the My-HiME manga is resurrected with the other HiMEs from the manga, she tries to bend Miyu to her will using a gravity altering device, but this fails. Instead of separating Miyu and Alyssa Kruger as Searrs desired, the two manage link hands after a struggle against the increased gravity, restoring Miyu's old memories and "unlocking" a new form of the Mythril Dress for Miyu. With this new power, Miyu attacks Alyssa Searrs, who becomes the first of the HiMEs to be defeated.
In the first episode of the OVA Miyu watches proudly as Arika destroys and incoming asteroid but springs into action after seeing a piece of debris that causes her to frown. When Shizuru Viola goes to investigate a strange crator a petrified Miyu falls from the sky and urges the Otome to run before Shizuru herself is swallowed by the shadow. This entity appears to be able to mimic the appearance and powers of anyone it comes into contact with, including Miyu, who it is seen imitating the appearance of before entering Garderobe. The second episode of the OVA implies that the enemy petrifies Otome by solidifying their nanomachines, and it is assumed this is what happened to Miyu as well.
Miyu is restored to normal in the final episode, following the defeat of Yuna.