Arcueid Brunestud

One of the heroines in Tsukihime, and probably the strongest, most incorrigible character in the story.
She is a True Ancestor, born in the 12th Century for the sole purpose of hunting those True Ancestors that had fallen and become demon lords.
After flushing out all of the Fallen Ancestors, she was commanded to start hunting Dead Apostles as well. However, when Roa tricked her into drinking his blood, she went berserk and wiped out most of the few remaining True Ancestors, prompting her to seal herself within the reality marble known as the Millennium Castle Brunestud. From that point on, she left the castle only to hunt down and execute Roa every time he reincarnated.
She is the princess of the True Ancestors, and used to have much longer hair than she does now. The reason for its current length is not because she cut it herself, but because it was stolen from her during a battle with the Dead Apostle Altrouge.
She was originally conceived of as a “pure and noble vampire,” but this concept seems to have been altered at some point in time. The responsibility for that lies entirely with the character designer. After seeing the smiling face he drew, even the scenario director's mental image of her began to change.
Of course, as a result she became the airheaded funny vampire everybody knows and loves, so it worked out all right in the end. I actually kind of like her smile the best out of everyone.
By the way, this is a secret but even though she's the main heroine she actually showed up the least out of everybody.

Tsukihime Data Collection: Tsukihime Dictionary
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A princess of the True Ancestors born solely to hunt fallen True Ancestors and the 27 Dead Apostle Ancestors. Usually she slumbers in Millenium Castle Brunestud which she materialized herself, but when a Dead Apostle shows its face, she awakens accordingly. When she finishes her hunt, she returns to her slumber, so since being born, she has spent less than a year's worth of time actually awake and active in the world (although since meeting Shiki she has been awake continuously for a year). In the visual novel Tsukihime, Arcueid meets Shiki, and awakens Shiki's demon hunter blood. Arcueid is a True Ancestor, and thus should basically never be defeated and killed, but Shiki's Mystic Eyes of Death Perception proved to be too much, and she died after being cut into 17 pieces. Despite not appearing at all in some of the routes in Tsukihime, Arcueid is undoubtedly considered to be the main heroine of the story, and she has never once given up her position at the top of TYPE-MOON character popularity polls. Like the proper heroine that she is, her catch phrase is “You killed me. I'll have you take full responsibility now.”

MB Official Strategy Guide: Melty Blood Glossary
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One of the heroines in Tsukihime, and probably the strongest, most incorrigible character in the story.
She is a True Ancestor, born in the 12th Century for the sole purpose of hunting those True Ancestors that had fallen and become demon lords.
After flushing out all of the Fallen Ancestors, she was commanded to start hunting Dead Apostles as well. However, when Roa tricked her into drinking his blood, she went berserk and wiped out most of the few remaining True Ancestors, prompting her to seal herself within the reality marble known as the Millennium Castle Brunestud.
From that point on, she left the castle only to hunt down and execute Roa every time he reincarnated.
She is the princess of the True Ancestors, and used to have much longer hair than she does now. The reason for its current length is not because she cut it herself, but because it was stolen from her during a battle with the Dead Apostle Altrouge.
She was originally an existence with no use for emotions, but after meeting (being killed by) Tohno Shiki her state of mind changed dramatically. It's similar to a broken machine that just isn't quite the same when you put it back together.
As a result, the pure and noble vampire princess was, through some mistake, reborn as a carefree, airheaded funny vamp who enjoys taking spontaneous strolls around town. Of course, there are two sides to any coin, and emotion is no different—now when she gets scary, she gets really, really scary. Luckily, despite her apparent optimism, she is actually a pessimist at heart, and so very rarely gets seriously angry.
Because even direct sunlight is just “a little hot” to her, she is free to wander around town as she pleases like some kind of cat, and has even been known to go to school just to bother Shiki and tease Ciel.
In Kagetsu Tohya and Melty Blood, a fragment of the reason for her birth was revealed.
Deep beneath her conscious mind lies a guiding principle known as the “Crimson Moon,” which very rarely expresses itself through her surface personality. This is the true identity of the vaguely self-important Arcueid that appeared when Shiki invaded Arcueid's dream (in Kagetsu Tohya), and who confronted the 27 Dead Apostle Ancestors (in Melty Blood). She is colloquially known as “Princess Arc.” (Since Berserk Arcueid is normally just Arcueid rampaging around in a frenzy, she's a special case.)
Though she is unmistakably the main heroine of Tsukihime, she doesn't get the amount of screen time you'd expect. It's really quite troubling.
Also, this is just a digression, but the “Nya!” she makes when you do her 5C (charged) attack in Melty Blood Re-ACT is a must-hear.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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Royalty of the True Ancestors. The Last Princess.
Though she has no ties to Japan herself, she came to Misaki in pursuit of a certain vampire.
She is a True Ancestor, born in the 12th Century for the sole purpose of hunting those True Ancestors that had fallen and become demon lords. She was originally an existence with no use for emotions, but after meeting (being killed by) Tohno Shiki her state of mind changed dramatically. It's similar to a broken machine that just isn't quite the same when you put it back together, probably. (Of course, that's leaving aside the question of whether or not she gained a new function or lost an old one…)
At any rate, the pure and noble vampire princess was, through some mistake, reborn as a carefree, airheaded funny vamp.
As the bastard child of the planet, her powers are totally ridiculous. Even sunlight just makes her feel a little listless, and the source of her energy supply is the earth itself.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary