Altrouge Brunestud

Vampire of the Dead Apostles. Mix of True Ancestor and Dead Apostle.
Can be called Arcueid's big sister. But her look is that of a 14 year old girl.
Usually, she has no special powers, and can't utilize Marble Phantasm.
But she can, like a magical girl, transform in two stages or something like that.

Brunestud [Term]
Despite being Arcueid's last name, strictly speaking it is actually more like a title. It is possessed by all of the members of True Ancestor royalty.
The Millennium Castle Brunestud where Arcueid had sealed herself away is not her original creation, but was actually created in the past by the most powerful True Ancestor, using marble phantasm. That original lord of Castle Brunestud died long ago, but ever since those True Ancestors capable of forming the castle have been uplifted as royalty. This is why the True Ancestors, who have no concept of hierarchy, nevertheless call Arcueid their princess.
After being tricked by Roa and killing most of the True Ancestors, Arcueid crucified herself on the throne of the castle she materialized herself. A thousand chains stretched from the castle's outer walls to the throne, so that she may seal herself away forever.
In Melty Blood, the Tatari Arcueid materialized the Millennium Castle. Later, in Re-act, it also appeared in her Arc Drive.
Currently, there are only two bloodsuckers with the name of Brunestud: Arcueid and Altrouge.

Crimson Moon [Term]
Another name for the organism that was used as the template for the True Ancestors. Also known as the Crimson Moon Brunestud.
After he vanished, individuals close to him in power started being honored with the title of Brunestud.
However, in the long history of the True Ancestors, only two have ever received the name.

Roa [Person's name • Dead Apostle]
Full name, Michael Roa Valdamjong. A Dead Apostle.
Though the Dead Apostles themselves merely see him as a normal Dead Apostle, the Church counts him among the 27 Dead Apostle Ancestors. Known as the Infinite Reincarnator, he attempted to realize an immortality fundamentally different from that of the other Dead Apostles. Originally he was a priest of the Church, but for the sake of surpassing his limits he had no choice but to become a vampire.
He tricked Arcueid, the strongest True Ancestor at the time, into drinking his blood, and immediately became a powerful Dead Apostle. Then, by unifying the factions of those Dead Apostle Ancestors who had already been sealed by the Church, he established his influence. In response, Altrouge set out to put the newcomer Dead Apostle in his place, but by that time Roa was already powerful enough to defeat her instead. Of course, it was only natural. Not only was Roa an excellent magus in his own right, but he was able to wield the power of the strongest True Ancestor, Arcueid, at the same time.
As a result, the 27 Ancestors left Roa to his own devices. It wasn't until many years later that he was finally defeated by Arcueid and the Church working together. After that, he reincarnated into new hosts a total of seventeen times, locked in a never-ending deathmatch with Arcueid.
Similar to Nrvnqsr, the identity called "Roa" was fading even as he clung to eternity.
Though the Roa that appears in Tsukihime has a rather brutal personality, that's because he's using Tohno Shiki as his base. The original Roa was a calm intellectual.
…Well, he was still cruel and utterly selfish at heart.
He was obsessed by his thesis of eternity and was convinced he needed nothing else. However, the moment he laid eyes on Arcueid he was captivated, even though he should have seen her as nothing more than a tool. After that, Roa's plan spiraled totally out of control… though he himself never noticed.

Arcueid Brunestud [Person's name • True Ancestors]
One of the heroines in Tsukihime. 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.

The Twenty-seven Dead Apostle Ancestors [Term]
The 27 ancestors who are the original source of the Dead Apostles. Currently, about half of them are being sealed by the Church.
As the 27 oldest Dead Apostles, there are those among them that have already disappeared. The reason that they are still counted as the "27" Dead Apostle Ancestors despite that fact is because whenever an Ancestor dies, one of his subordinates will inherit the position.
However, although proper Dead Apostles will create subordinates and work to expand their influence, there are some Ancestors who have no interest in any of that at all. Those Ancestors that used to be magi before they became Dead Apostles tend to be more akin to "seekers" rather than "bloodsuckers". In that sense, both Roa and Nrvnqsr could surely be said to have been brothers.
Though it isn't really related to the main story of Tsukihime, below is a brief introduction of the 27 Dead Apostle Ancestors. The missing numbers represent those Dead Apostles that have either been executed by the Church, or are currently "atoning".

1/ Primate Murder
Details unknown. Murderer of primates. The White Beast. Altrouge's demonic dog (the Beast of Gaia).

2/ The Dark Six
The very first Dead Apostles. The six royals of darkness. Currently resuscitating.
It is said that their revival will unite the 27 Dead Apostle Ancestors, but not a single Dead Apostle knows their true colors.

3/ Crimson Moon Brunestud
Ultimate One. Currently vacant.

4/ Wizard Marshal Zelretch
One of the five existing magicians. Also called Zelretch of the Jewel.
Despite being a Dead Apostle, he cooperates with the True Ancestors. Arcueid thinks of him as a sort of grandfatherly servant. He toppled the Crimson Moon in battle, but had his blood sucked at the last second and became a Dead Apostle. However, even after becoming a vampire his position as a magician stayed intact, and he traveled around the world to various countries where he would take disciples and uniformly ruin them all.

5/ ORT
Details unknown. Seems to be a mutant species that crash landed in South America sometime before the Common Era. As an attack-type organism, its power is on a completely different level. It uses the encroaching reality marble, Crystal Valley. Its appearance is similar to a spider's. The former 5th capture it, and was instantly annihilated. Afterward, it was confirmed to possess vampiric traits, and is now treated as an Ancestor itself.
Type Mercury.

6/ Rizo-Vaal Strout
One of the longest-serving Dead Apostles. Black Knight Strout. One of Altrouge's bodyguards. On account of Apostle recklessly tried to being inflicted with a curse of time, he is immortal. The True Devil Neardark.

7/ Disemboweling Forest Einnashe
A bloodsucker formed from carnivorous plants. It is a forest unified with a single will, and is very similar to a phantasmal species. Once every several centuries, a deep crimson fruit will mature deep within it. This fruit is the congealed mass of all the blood Einnashe took in over the years, and legends state that it has the power to grant immortality to whoever eats it.

8/ Fina-Vlad Svelten
White Knight Vlad. One of Altrouge's bodyguards. The Vampiric Count. Devil of Stratovaris. Captain of the Ghost Fleet. He has an interest in handsome youths, and only drinks the blood of other men. He and Rita simply do not get along at all. Possesses the reality marble Parade.

9/ Altrouge Brunestud
Hybrid of a True Ancestor and a Dead Apostle. Vampire princess of the Dead Apostles. "Black". Master of blood and contracts. Primate Murder's owner. Appears to be a 14 year old girl.

10/ Nrvnqsr Chaos
A living mass of chaos. A magus whose research ultimately led him to become a bloodsucker.

11/ Stanrobe Calhin
The Duke of Predation. Wraith. Also known as the "City Devourer". An invincible character whose density of existence will reach zero in about 200 years.

13/ ?????
Tatari. A Dead Apostle purported to exist. Only the name "Night of Wallachia" remains as a legend handed down through the ages.

14/ Van Fem
One of the longest-serving Dead Apostles. Dark Lord of the Business World. Puppet Master. He created seven great golem castles, and so is also known as "Van Fem of the Demonic Castles". A man of the world, who holds a prominent position in public society, and even worries about global affairs. The fifth of his castles, Matri, was captured by Parade during an engagement with the White Knight Vlad, and ever since then he has harbored a grudge against Altrouge's faction.

15/ Rita Rozay-en
A self-proclaimed artist. Indulges in a distinctive hobby even among the Dead Apostles. A second-generation Dead Apostle that inherited her castle from the Dead Apostle that made her, but since she was the official successor her situation is not the same as Enhance's. The very model of a noble bloodsucker, she indulges in a hell of debauchery and luscious depravity day in and day out.

16/ Gransurg Blackmore
Peng. The Black Wing Lord. A Dead Apostle known as the Moon Eater. Uses the reality marble Nevermore. A magus whose research ultimately led him to become a bloodsucker. Despite being a Dead Apostle himself, he possesses an ability that is strong against other Dead Apostles.

17/ Trhvmn Ortenrosse
One of the longest-serving Dead Apostles. The White Wing Lord. A magus who became a bloodsucker. The quintessential vampire. King of the Dead Apostles. As the official representative of the 27 Dead Apostle Ancestors he has the greatest influence – formally, at least. In practice, however, Altrouge's influence is even greater, and the two are constantly at odds. Also, Trhvmn was the Dead Apostle who proposed they start hunting True Ancestors. In a sense, he was the reason Nrvnqsr met his end in that far-off Eastern land.

18/ Enhance
The Knight of Vengeance, Enhance. A Dead Apostle who killed his master, and only just took over the original 18th Ancestor's position. Consequently, he is known derisively as the "Single-edged Enhance Sword".

20/ Merem Solomon
The Great Four Demon Beasts. Despite being a Dead Apostle, he is the fifth member of the Burial Agency.
Goes by the alias of Crown. For Ciel, he is something like a pesky senpai. A collector of treasures from all times and places. Probably the only reason he joined the Burial Agency is because he wanted to be close to the treasures sealed by the Church.

21/ Sumire
Aquatic Demon Sumire. Water Bottle. An oddball Dead Apostle that lives underwater. Traditionally, vampire folklore states that they cannot cross running water.
Rivers and oceans are like impassable barriers to them. Sumire has conquered this weakness, but in exchange it seems that it has become hard for her to move on land. Also, she's perpetually drunk. While this may make her seem pretty weak, in reality she has a strong enough compositional ability to break into the upper echelons. Among the Dead Apostles, she is the only one capable of using Marble Phantasm. If you put her on dry land and dry her out, her drunkenness will disappear and she'll be able to demonstrate her true power, maybe. Her outward appearance is that of an easy to get along with drunk lady, and she has a castle, but it's a completely abandoned husk. There's a rumor that she and Rita are on such good terms they could just kill each other.

24/ El Nahat
Background unknown. The artifact called the Book of the Stomach World. A peculiar vampire embodying refraction. Possesses the unique ability to almost certainly erase an opponent in one-on-one combat, but his own body is destroyed every time he uses it and requires several decades to revive. Thanks to that, he is currently sealed by the Church. Regarded as the Burial Agency's ultimate anti-Dead Apostle trump card, they only bring him out on rare occasions.

27/ Caubac Alcatraz
Dead Apostle of the Millennium Lock. A magus whose research ultimately led him to become a Dead Apostle. He was also a believer, just like Merem and Roa. He constructed a labyrinth to protect the culmination of his ideology, the Holy Scripture Triten, from outside invaders. However, he accidentally trapped himself inside, and so has not made an appearance for several hundred years.

EX/ Michael Roa Valdamjong
The Serpent of Akasha. The Infinite Reincarnator. Though the Church counts him among the 27 Dead Apostle Ancestors, the Ancestors themselves despise him as an "extra number".