Matri

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".