|“||If it weren't for him, I...probably wouldn't be here. In fact, I think it's because of him that I feel this way. I don't care if it feels wrong, I don't care about the technicalities or face-value facts. The truth is, I... love my big brother. The one who has my heart... is Issei, the one who gave me a reason to live.||”|
–Mumei's Thoughts on her Relationship with Issei
Originally from an unknown family, Mumei was abused by her parents before their untimely deaths at the hands of an unknown assailant wielding the Demon Sword, Murasame, which would later come into her possession. Through out the rest of her life, she would become the primary target of bullying all the way to her second year of middle school, where she would come to be friends with the soon-to-be Perverted Trio. Due to Issei's interventions, and later his parents, she would come to be adopted by his family, and became her foster home.
Mumei is one of the few recorded individuals who is able to resist the influence of Murasame, which sends anyone who wields into a bloodthirsty frenzy. Additionally, she is a practitioner of the Hiten Mitsurugi, which was taught to her by a descendant of Himura Battousai, Kenshin Himura.
Appearance-wise, Mumei is known for her cute short, bob-cut black hair, soft brown eyes, glasses and her rather large breasts. She is usually seen wearing her Kuoh Academy uniform, but is shown wearing a black vest with a pink dress underneath outside of school.
Initially, Mumei is very introverted and quiet, rarely speaking to anyone for any extended period of time. She is shown to be rather submissive and tends to "go with the flow" even when someone is harassing her. This timid demeanor stems primarily from the trauma she received when her parents died. However, though her interactions with Issei, it served to pull her out of her emotional shell and she becomes much more expressive and shows a caring side to those close to her.
She is very close to Issei, likely due to him being the only person to ever care for her, though her relationship with him is much more complex. Mumei is actually romantically involved with Issei, despite the fact that they are apart of the same family, if only in name and not by blood. She loves him dearly to the point where she is somewhat obsessed with him, though she claims the reason for this is because Issei was the one to be the first person to ever care about her, something she never experienced, not even with her parents, and was her very first friend. His devotion to helping her led to her falling for him. In a way, her relationship with Issei is similar to Akeno's in the canon story, where Issei serves as emotional support for her.
That said, Mumei can be very aggressive and volatile when push comes to shove. If Issei, or any person she holds dear to her, is threatened, her timid persona is gone, and is replaced with a terrifying warrior that will do anything to protect the people she cares for. This side of her was forged through her training with Kenshin as he taught her the Hiten Mitsurugi Style swordplay. She is fearless, and without mercy, as she killed Raynare when she tried to kill Issei after revealing her true nature to him, even referring to her as a "filthy angel." She is especially concerned about Issei, as she will do anything, even so far as to risk her own life, to protect him.
Mumei's story is a rather tragic one. Since being born into the world, she has lived a life of constant abuse, be it at school, or at home. Her parents were abusive, having a drunk for a father and a prostitute for a mother. She also had several bullies at school after her as well, and none of the teachers helped her at all. One day, during her third grade year, the abuse had reached the point where her father had tried to strangle her, and her mother did nothing but watch. Thankfully, or unfortunately, however one is to look at the situation, both her father and mother were killed at the hands of an unknown assailant wielding a blade wreathed in a demonic red aura. The killer, after cutting down her parents, approached her, looking ready to kill her as well, but hesitated. Afterwards, the killer stabbed the blade into her chest, though it appeared to do nothing to her. In fact, there was no wound in her body at all.
After the incident, Mumei left the household, and began living by herself, keeping the mysterious blade with her. She claimed that she felt an odd connection to the sword, and couldn't leave it behind, and in order to keep it with her, wrapped it in various pieces of cloth in order to conceal it. As she was living on the streets, she naturally became accustomed to living in small homeless shelters, and accepted the help of others, but never seriously interacted with anyone. She also remained unresponsive to the continued bullying, as she continued to go to school. However, it was around this time that she soon came to encounter three young boys by the name of Issei, Matsuda, and Motohama, who were considered to be the most perverted boys in school. As it happened, Mumei was being pushed around by a much larger student, and was thrown to the ground when Issei and his friends stepped in to help her. This became the first time someone ever tried to help Mumei, and surprising her further was the fact that Issei asked if he could be her friend.
Since becoming friends with Issei and his group, Mumei was slowly, but surely, becoming a bit more defensive against her bullies. Issei had also taken to asking her to come home with him a few times when he saw that she had not been eating recently, which had led to her becoming close to his parents as well. It hadn't been until during the summer break after graduation that Mumei's parents were finally discovered after almost two years since their passing. When Issei learned that she had been living out on the streets by herself, he had all but made it clear that she would be staying with him and his family. Much to her shock, and secretly, to her joy, his parents agreed to look after her. By the start of their first year in high school, Mumei had begun to live in the Hyoudou household, where she had become even closer to Issei.
At some point in time, Mumei and Issei became romantically involved, though the latter admits that he was stunned and left a little off put but her affections, but he also confessed that among the reasons he helped Mumei was because he had fallen for her. She is aware of his desires of becoming a Harem King, and while she does not support, nor like the idea, she allows him to see other girls, so long as he continues to love her. Several months prior to the story, Mumei had begun receiving lessons from a wanderer named Kenshin Himura, who was in search of a sword known as Murasame. When Kenshin recognized her carrying the blade he was looking for, he explained that Murasame was a legendary blade forged by a master swordsmith, but held a cursed power that made anyone who held it crazed with bloodlust. Seeing that she was able to somehow resist it's influence, Kenshin offered to train her. At first, Mumei refused his offer, but upon recalling an incident where she caught Issei and his friends being beaten by Katase and Murayama, she accepted, wanting to protect him from any harm. As a result, she became the first student in over a century to the Hiten Mitsurugi style, which was used by Kenshin's ancestory, Himura Battousai, also famed as the man who brought forth the Meiji era; "Hitokiri no Battousai."
- Mumei's appearance, personality, and backstory are all based off of Anri Sonahara, from the popular mangika and light novel series, Durarara!!
- Mumei has been hinted to have the largest bust out of all the girls in Kuoh, and as a result is famed as one of the Great Ladies of Kuoh Academy, despite being in the same year as Issei.
- Mumei is one of the few characters in the story to not be apart of any faction or organization. She is purely on her own, and is aided by fellow soloists.
- Mumei is '5, 6" ft tall, and weighs approximately 128 lbs.
- Her favorite food is Tofu.
- Her likes include Issei, cute things, pandas, teddy bears, and her dislikes include those who would try to take what is most precious to her, getting cut while cleaning her sword, and someone teasing her brother.
- Her hobbies are listening to pop jazz, reading teen romance novels, and (perhaps as a result of being in a relationship with Issei) reading brother/sister incest doujinshi.