Manipulation of darkness
Current incarnation, over 1000 years old, assumed to be close to 2000. Exact age is unknown. Her soul has existed since near the beginning of time.
She has red eyes, and long blonde hair. She almost always wears black and white, and usually wears a red ribbon somewhere on her body. She has the appearance of a human in her early 20s, but she physically altered her body with her abilities to keep it to her liking.
Shinki (Wife (Young Rumia))
Arche (Friend and servant (Young Rumia))
Kana Anaberal (Wife (EX Rumia))
Lydia Anaberal (Daughter)
Adele Anaberal (Daughter)
Utsuho Reiuji (Friend (Young Rumia))
Kasumi Reiuji (Daughter)
Serene Melancholy (Granddaughter)
Mima (Biological Mother)
An unknown youkai with powers from a moon god (Biological Mother)
Louisa (Spiritual Mother)
Millions of years ago, when the universe was first created, there were two gods. One of them, Louisa, became evil, and created what eventually became the youkai of today. She had only one biological daughter, named Rumia. At one point, Rumia sealed away her mother with the help of the dragon god and possibly some others. In the millions of years since then, Rumia has been reincarnated a number of times.
Rumia's current incarnation was born over 1000 years ago like any youkai, and was seemingly normal for the first few years of her life. At the time, she was carefree and incredibly friendly by normal youkai standards. When she was about 4 years old (Having the appearance of being a human child of about 7-8 ), the true power of her abilities were discovered by her mother. Afraid of her sheer power and the possibility that she might destroy them all, she tried to kill Rumia when she was young. However, Rumia killed her instead. It was assumed that Rumia killed both of her parents, but one of them had left a few years before and was never heard from again. No one alive knows who Rumia's real mother was.
Afraid and confused, Rumia ran away from everything she knew and hid in the forest, where she lived for a few years of her life. However, everyone who learned about her abilities was equally scared of her, and many others tried to kill her. Over the years, Rumia's fear slowly turned into hatred, and she started taking pleasure in hurting the people that feared her.
Like most youkai, Rumia fed on humans. When she was about 50 years old, most humans and youkai thought Rumia was only a myth and that no youkai could really be that powerful. Every night however, Rumia would pick off a human or two from nearby villages to feed on for that night. Many arrogant humans and youkai who believed she was nothing more than a youkai who hid in the shadows foolishly went into the forest, seeking to kill her, but few ever returned. Those who did return never spoke of their attempts to kill Rumia.
Over the years, Rumia's sanity slowly diminished, and when she was a few hundred years old, she left her forest and began killing anyone she came across. Eventually she started killing anyone she seemed a threat, wiping out all of the satori (except for Satori and Koishi), and most of the oni, causing them to flee to the Underground. Yukari Yakumo tried to stop Rumia, but wasn't strong enough to do it herself, and asked Yuka Kazami, the strongest youkai currently living in the area other than Rumia and Yukari, for help. The two of them fought against Rumia and eventually succeeded in defeating her, but not before both of them were near fatally injured in the process, forcing Yuka to sleep off her injuries for over a thousand years.
Yukari felt bad that Rumia's powers were the cause of her insanity and the problems in her life. Instead of killing Rumia, Yukari sealed her powers within a ribbon in her hair and returned her mind to its childish state from before her parents tried to kill her. This accidentally split Rumia's personality into two, the insane part that lay dormant, and the childish part that was currently in control.
For the next thousand years, Rumia was just another fairly harmless youkai that lived in Gensokyo. She would occasionally feed off of humans from nearby villages, but most knew how to avoid her and how to defend themselves against her.
Eventually, Rumia met Shinki, the creator of Makai. They eventually became very close, and eventually Shinki took off Rumia's ribbon. Because of her close relationship to Shinki and the extremely large amount of time her powers and other personality had been sealed for, Rumia retained her childish personality. While her powers greatly increased, she was still nowhere close to her full potential at the time. Eventually, Rumia and Shinki had a child together named Nisshoku. One of their servants, a young fairy named Arche, eventually became close friends with Rumia as well.
Shortly after Nisshoku was born, Shinki had to leave Makai to help with an incident involving her step daughter, Alice Margatroid, and she left Rumia in charge of Makai. With Shinki gone, Rumia's other personality started to take control of her. The poltergeist Kana Anaberal entered Makai as well, seeking a new place to possess. Rumia allowed her, intending to use Kana's powers over the Void to help her control Gensokyo.
Over a period of a few months following the time Kana entered Makai, Rumia began terrorizing Gensokyo again, mostly by taking one or two humans a day from the village, as well as taunting Reimu and anyone who even considered trying to stop her. When Yukari began to get involved however, Rumia became more cautious. She would often hide in Reimu's shadow in order to overhear any plan her and Yukari might come up with.
Using Kana's powers, Rumia threatened Reimu telling her that if she or anyone else did anything to stop her, Kana would use her powers to send all of Gensokyo to the Void. With Kana's help, Rumia also permanently eliminated anyone who tried to stand against her, including Letty and Kotohime.
However, with the help of a succubus named Lilith, Reimu managed to keep the Human Village safe from Rumia. Angered by this, Rumia killed and ate Lilith's siblings, Alan and Melony, in order to get back at her. Lilith then used her powers to turn the bodies of all the humans (whose souls were kept safe, out of their bodies) into energy, absorbing it to become powerful enough to stop Rumia, as well as nearly destroying Kana. Having gone completely past the point of insanity, Rumia used all her power to destroy the balance of Light and Darkness, thus bringing the Cloud of Darkness into Gensokyo in a last ditch effort to defeat Lilith. The Cloud of Darkness turned on Rumia however, due to her being the one to destroy the balance in the first place. Although nearly destroyed, Kana used the last of her energy to keep Lilith and the Cloud of Darkness off for a short time, allowing Reimu and Rumia to escape. After they had left, both Kana and Lilith were consumed by the Void.
After losing Kana, Rumia felt deep regret for bringing the Cloud of Darkness to Gensokyo so recklessly. She decided to help Reimu return everything to normal, and teamed up with her in the meantime. However, a copy of Utsuho Reiuji that had been sent to the Void a few months before managed to escape when the Cloud of Darkness came to Gensokyo, and had additionally managed to gain some of the Void's power. She used her new power to find the Cloud of Darkness and fuse with her, making both of them more powerful than ever. Utsuho fled to the Void, drawing Rumia, Reimu, Miasma, Sakuya, and the others in to fight her.
While inside the Void, Rumia and Sakuya were cut off from the rest by the Cloud of Darkness, who Utsuho had sent a projection of to separate their group. While the two of them stayed behind, the rest of the group went after Utsuho. Miasma was near fatally injured in the battle, but eventually they succeeded and Utsuho was defeated, causing her to split from the Cloud of Darkness. Having regained her senses, Utsuho managed to reason with the Cloud of Darkness, and she returned everything back to normal that Rumia had used the Void to destroy.
With Kana now back to life, Rumia settled down with her in Makai and had two twin daughters, Lydia and Adele, with her. For the next few years Rumia no longer caused any major trouble, until she met Hana, who could control light and was Rumia's opposite. She tried to kill Hana, but Miasma managed to sneak up on Rumia and bash her skull in. Injured but not dead, her consciousness went into a coma and her younger self was able to take control of their body again.
With control over her body again, the younger Rumia asked Yukari to split her body in two, so that her evil half would never be able to suppress her again. She agreed, but Rumia's evil half was still unconscious. Kana found out, and took her body away and hid with it along with Adele in an unknown location, while Lydia had decided to settle down with Miasma.
The good Rumia returned to Shinki in Makai, and resumed her normal life. Through a mishap likely caused by the lust youkai Anita, she accidentally had a daughter with the original Utsuho Reiuji, named Kasumi. Despite this, Rumia preferred not to get involved in major incidents unless they directly concerned her.
In order to get revenge on Miasma for what she did to her mother, Adele absorbed Kana and created Devita, as well as adopted Anita's daughter, Annette, after her death, not knowing that Miasma was the other parent. When Rumia woke up a few months later, she forcibly separated Kana and Adele, and let Lydia and Miasma live their lives in peace. Annette and Devita both moved into Mugenkan with them, and Rumia and Kana returned to Makai. Adele, feeling like she could no longer trust anyone, left Gensokyo to an unknown location.
Despite causing some minor trouble on occasion, the newly awakened Rumia now mostly kept to herself. She used her powers to spy on all of Gensokyo for her amusement, and gave up on trying to kill Hana. She even helped Reimu against the sin youkai in a few instances, usually for a price however. (Such as tricking Reimu into having sex with her to save Yorihime from the sin youkai's ship.)
After Louisa was released from Eden by Reimu, she contacted the evil Rumia and the two came to an agreement. Louisa was angry with Rumia's past self for helping to seal her away, but she seemed quite pleased with how her current self had turned out. In addition, Rumia agreed to help her by keeping Hana and the dragon god out of commission so that no one would stand up against them.
(tl;dr, evil Rumia is a b***h.)