Arcueid Brunestud

The eponymous main heroine of Tsukihime. So far, has only appeared in the Christmas wish-fulfillment match of Season 5.

Princess of the Crimson Moon
When Gaia was being besieged by humans, it understood that the interests of humans and the world had since parted ways. Unable to judge itself, the earth cried to be judged by other celestials. The moon responded, asking in return for the Ultimate One to be allowed to live on it. The Earth formed beings based on his image to defend itself, True Ancestors.

The Crimson Moon's Ultimate One, Brunestud, had in mind plans to make Earth his kingdom, but being an strange body to the planet, even with it's will, he foresaw his eventual elimination. He implanted his ability to bring Millennium Castle Brunestud into his "descendants" so that when the time came for a pure vessel strong enough to use it to be born, he would be reborn and accepted by the world as Ancestors are a byproduct of it.

After his demise, eventually an Ancestor pure enough to heir the name Brunestud was created, Arcueid, regarded as the princess of the Ancestors. Being based on the Crimson Moon, Ancestors not only inherited his abilities, but are also inherently flawed with his psychological need to drink blood. This "need" will grow exponentially, eventually descending them into madness if the Ancestor doesn't try to contain it or fall into slumber to suppress it. Due to this, Ancestors seek to make a second generation of Vampires as slaves to consume, the Dead Apostles. However, their flaw became heightened, besides lowering the severity of the madness, into not only a psychological need to consume blood but a biological one. Eventually they rebelled, and at this point Arcueid was used against Apostles and maddened Ancestors, and then wiping her memory, without ever caring of slaying her own due to this.

When the scholar Roa Valdamjong seeked to attain immortality, he discovered Arcueid and tricked her into drinking his blood for the first time, causing her to go berserk and massacre all the other Ancestors, and having her power stolen by him. After regaining her sanity, shocked, she chained herself to the throne of the castle and fell into slumber.

Eventually, she awoke and destroyed his body, but he had already attained the secrets of reincarnation. She then awakens every time to destroy him, but with her increasing bloodlust, she needs to use 70% of her power at a time to contain it.

Season 5
Roa might have reincarnated again, and he might try to attain power in TFR. Will Arcueid come to save us?

Season 6
Visited a nearby hot springs with the other TFR females... and the Aspect Rean duo.

Trivia

 * On the episode she debuted in, OP had made a roided monstrosity of a CAW for Arcueid that caught everyone off-guard. OP fixed her next episode, and the first creation was nicknamed "Orcueid".