She works on diplomatic relations while her husband runs internal affairs.
During the appearance of waves, she attended the world conference in regards to fight the waves and to summon the heroes. It was accepted that her nation would be fourth in order for summoning the heroes and also specified that each nation supposes to summon one of the four heroes and that too in presence of all the world leaders. Though they summoned in a particular country, the heroes are to travel the world and fight off the waves. These were the few conditions which were agreed upon.
But to the shock of everyone including herself, Melmorne already summoned all the four heroes which is news to herself. It seems the Church of the Three Heroes took advantage of the queen's absence, to summon the heroes. The conference was in turmoil but she kept her face. Assassins were send against her but were neutralized by her own. That led to a lot of criticism from other countries, it was the problem the queen was taking care of that lead to her delayed absence. She saved face by saying that any nation would have tried to monopolies the heroes and her nation was just trying to save them-self first and added that it was not wise to attack the nation which is in possession of four holy warriors. Mirellia proposed an idea of sharing the heroes with other nation by sending envoys to meet and give them an offer so they may move to their nation.
But what happened next was a surprise for herself and her second daughter, Melty who was accompanying her in diplomacy. All the heroes refused to meet the envoys, including the shield hero whose treatment was more worrisome. While Melty was puzzled by this development, Mirellia guessed what was going on. Again every nation blamed her of breaching the agreement, as they were preparing for a hero welcome to their nation and left with much disappointment. The Demi-human nation, which worshiped the shield hero was furious at her. Mirellia explained that all the heroes found scums in her nation and were currently at the removal of filth there. She also brought to the attention of how the last shield hero died in Siltvelt and told them to not point fingers at her.
Knowing that there was no turning back, she wrote numerous letters to her husband emphasizing the need to treat all heroes equally and to take special care of shield hero. She gained the support of Fulbert, by certain promises and hopes to find a way around it.
At first, she was more concerned with the ruckus caused by the shield hero, such as attacking others with balloon monster, the trouble stirred at the dining center and also his way of avoiding anyone getting close to him, including the Demi-humans and others who favors him. By the information given to her by the shadows, she summarised the reason for his distrust. She concluded that her husband is not going to comply with her words and decided to take action on her own. She managed to secure a slave as Naofumi's comrade with great difficulty. She kept on sending soldiers with letters, advising her husband to treat all the heroes equally and to take special care of the shield hero, but it was in vain. Mirellia also learned about her husband and elder daughter, Malty misusing their power and lavish spending of countries treasuries to their own gain. They also started shunning all nobles favoring demi-human.
Mirellia was happy at the news of the subjugating of the wave by the heroes and the shield heroes effort in protecting a village, despite his poor treatment. She learned of Naofumi's reputation begin to rise. However, her happiness short-lived, as she was infuriated at her husband for arranging a rigged duel between the spear hero and shield hero to take away the slave of the shield hero. Aultcracy also treated heroes in a biased manner with Spear hero of most favor and shield hero being the least or none. This became a new reason of conflict with the neighboring nations and She worked to her bones with her diplomatic skill to release the tension. She also burned the portraits of her husband and first daughter to release her own stress. Her letters remained unanswered by her husband and his action towards shield hero got worsened. He also ignored the messengers sent by her. By this time her husband named the spear hero as a lord of the wave affected village, where Malty levy heavy taxes under the pretense of rebuilding. The queen sends her shadows warning her to back down. Not wanting to give up, Malty challenged the Shield Hero to race with the Spear Hero for the lordship of the village. Despite the cheating by the spear hero side, shield hero emerged victorious earning him goodwill among the people.
Mirellia also hears about the consequence of other heroes poorly thought actions, While the shield hero clearing all their messes, earning the trust of her peoples. She finally fell sick, due to overwork.
She became more furious, after her husband sent an irrelevant letter, wishing to see Melty. Reaching her limit, she begins to make her own plan with shadows against the Cult of three heroes, as she already kept tabs on their evil plot.
Melty was lost on the way and was helped by Naofumi to return to her home without knowing who the other person was. After learning their identity, She tried to negotiate with shield hero but was shunned by him for being the royal of the nation. After the third wave, Naofumi emerged victoriously. This led to a serious of incident starting with the rift between Naofumi and the Aultcracy widens farther, the Cult of three heroes became more anxious of the result and becomes desperate and the world diplomats were concerned of the heroes not co-operating with each other. Melty in hopes of mending the relation between Naofumi and his father, begins to chase after Naofumi whose was on his way to demi-human country for a class up.
As Mirellia hoped, the Cult along with Malty plans to execute Melty and pins it on Naofumi, so the Cult can save face in public for their teaching and Malty can get the throne. As Mirellia planned, Naofumi was forced to take Melty under his protection. Mirellia also orders the shadows protecting the Melty to arrest Naofumi if he became a threat to Melty. Mirellia was constantly informed about the situation and was surprised of knowing Melty getting close to Naofumi and was acting to her age. Finally, the Cult of Three Heroes showed their true colors and tried to execute all heroes with their companions and princess. Both Ren and Itsuki were saved by the queen's shadow in nick of time and informed of the Cult subjugation force coming to aid them. Finally, the four Heroes lined up to fight the pope, and Naofumi with the aid of the queen gave the finishing blow and saves the day.
As soon as she returned to the castle, she permanently deprived Trash and Bitch of the authority of the imperial family, as punishment for their evil deeds. She wanted to take special care of the shield hero for he may be the key for the everlasting peace between her and her rival nation, She even went on planning to marry her daughter to the shield hero to end the war and to attain the goodwill of Demi-humans and Siltvelt. But, this plan was ruined because of the action of her daughter, Malty and her husband. This is noted by her scolding to Malty, as she told that she might have even considered giving her throne to Malty if she had remained in Naofumi's party and seduced him rather than betray him. Now she held hopes for her second Daughter, Melty for the same idea.
Mirellia begs for Naofumi's forgiveness and asks for his help in saving the world, by staying in Melromarc. She tries to persuade him in every way, as his choice may lead to war. She was able to gain the trust of hero with the condition of it being the only time and magic contract of her being a slave if she were to betray him. She promised him better and preferential treatment for his cooperation. She cleared all of the fake charges against him, overlooked his own crime which was done due to circumstances and punished her husband& daughter for all crimes.
Naofumi accepts her to be a better administrator and realize that things might have been different if she were to run the things from the start of his arrival. He also realized that she could have handled the heroes more efficiently. He can also guess that she might have pushed her daughter to a political marriage with him without him even knowing it, under the guise of love marriage.
It was later revealed by Mirellia that her country lost access to vassal weapon wielders, since Melramorc already monopolized the four holy heroes. If those heroes were to be accessed than the four holy heroes might be able to form party with the vassal wielders instead of adventures.
Naofumi also came to respect her, after seeing the political instability in Glass's world. For she had prioritized the world's safety and quelled most of the problems which may make the same in her world. This gave a convenient environment for the heroes to face the wave unlike Kizuna's world.