Enhance

One of the 27 Dead Apostle Ancestors. The Knight of Vengeance.
A bloodsucker who only just became an Ancestor, he is known derisively as the “Single-edged Enhance Sword.”
Although normally Dead Apostles only fight within their own group, Enhance is hunting the other members of the 27 Ancestors themselves.
Even though he's a vampire, he is incomplete, and lacks the superhuman innate abilities possessed by the other Ancestors. On the other hand, his force of will and ability to take action is foremost among the Dead Apostles.
By nature, the immortal Dead Apostles are tranquil beings, but as Enhance is still almost human his vindictiveness knows no bounds.
He fights using the demonic sword Avenger, which he inherited from the Apostle he replaced, and a gun that was manufactured by the Church. Because of its holy origins, the gun corrodes his left arm each time he uses it.
If you're wondering what an entry like this is doing in this encyclopedia, it's because he got name-dropped in one of the routes, during a conversation between Arcueid and Ciel.

Tsukihime Data Collection: Tsukihime Dictionary
§

One of the 27 Dead Apostle Ancestors. The Knight of Vengeance.
A bloodsucker who only just became an Ancestor, he is known derisively as the “Single-edged Enhance Sword.”
Although hostility between Dead Apostles is nothing strange, that only refers to power squabbles within a group. In other words, while Dead Apostles will fight for the right to become their own Ancestor's successor, there is no conflict between the separate Ancestors' factions. However, Enhance is an exception to the rule, and is hunting the other members of the 27 Ancestors themselves.
Even though he's a vampire, he is incomplete, and lacks the superhuman innate abilities possessed by the other Ancestors. On the other hand, his force of will and ability to take action is foremost among the Dead Apostles.
By nature, the immortal Dead Apostles are tranquil beings, but as Enhance is still almost human his vindictiveness knows no bounds.
He fights using the demonic sword Avenger, which he inherited (stole) from the Apostle he replaced, and a gun that was manufactured by the Church. Because the gun is an artifact of the Church, it corrodes his left arm each time he uses it.
If you're wondering what an entry like this is doing in this encyclopedia, it's because he got name-dropped in one of the routes, during a conversation between Arcueid and Ciel.
If there's ever a Tsukihime 2, it's rumored that the story will revolve around Enhance. Though, the author gets bored really easily so maybe take that with a (huge) grain of salt, yeah?

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
§

Avenger. A member of the Twenty Seven Dead Apostle Ancestors. Number Eighteen.
He was given the title Blade (Enhance Sword) by those despised him. A Vampire who recently joined the ranks of Ancestors.
It was not unusual for Dead Apostles to wage wars against each other, and the purpose was to wrestle for power within the organization.
Usually the subordinates fought for the right to inherit the title of an Ancestor. Fighting against the other Ancestors was almost unheard of.
Enhance broke this rule, as he actively hunted for the other Ancestors.

Character Material: Prelude Explanation