Arach attended university with Makihisa Tohno and studied architecture alongside him. It was through this connection that she became the family’s resident doctor following Makihisa’s death. She still does architectural work as a secondary source of income alongside her career as a doctor.
She has some relation to the Twenty-seven Dead Apostle Ancestors, in particular the Rose Princess Rita Rozay-en as the serum she injected into Noel is called Idea Replica and gave her Rita's Mystic Eyes of Roses, implying that Dr. Arach's procedure somehow granted Noel some of Rita Rozay-en's other abilities, as she also gains the power to sprout thorny vines from her person after injecting herself with the final Idea Replica serum that Arach had given her. She also appears to have some unknown connection to Roa, as she displays knowledge of Noel's past, making comments regarding her psychology to Shiki after Ciel's battle with Vlov. It is also implied that Dr. Arach is the "Modding Maniac" mentioned in Vlov Arkhangel's profile, as well as the "certain Dead Apostle" who gave him a technique that allowed him to retain his sanity until the end.
Arach is a woman with light brown hair and amber eyes. She doesn’t wear makeup, and her hair is arranged in a very messy way. She wears a white lab coat with a yellow-and-black checkerboard pattern on the inside over an orange blouse, black skirt, black-and-orange tights, and black high heels. She has very large breasts, and wears her blouse with the top opened as to completely expose her bra and cleavage. Around her neck is a gold necklace with a red jewel attached to it.
An energetic scientist with a habit of skipping work. Everything Dr. Arach does appears to be done on a whim, and she refers to to herself as a "free spirit pulled away from her roots", saying she was a "spider without a web going wherever the wind would take her" before she met Makihisa. Her behavior is loud and hyperactive, with very little consideration paid to what those around her consider appropriate conduct, and all of her emotions greatly exaggerated. She displays very visible glee or disappointment when interacting with Shiki on medical business, whether it be treating wounds or administering vaccines of questionable origin. Due to her medical background, she will occasionally refer to biological characteristics or chemical interactions within the body when describing her emotions. She is also extremely sexually forward, talking about her own arousal in frank terms and wearing highly revealing clothing.
Below her comical exterior, however, a far darker nature is apparent. Akiha's borderline abusive upbringing was caused by her suggesting to Makihisa that strict education was a good stopgap measure to contain Akiha's miraculously powerful oni blood. In Ciel's route, she reveals herself to be involved with Dead Apostles, transforming Noel into a vampire through the means of an injection called Idea Replica. She also performs experiments on the students of Metropolitan Souya High School involving questionable injections, abusing her authority as a doctor and doing so under the guise of administering vaccines.
When introducing herself to Noel and administering her Idea Replica, Arach states she hates the story of DickensWP' A Christmas CarolWP as it concerns "someone strong turning into a weakling", and thus it is lacking in "love". She describes her “life’s work” to Noel as making the weak stronger, especially those who did not put in any effort themselves to become stronger on their own. She also emphatically refers to “love” multiple times during her first meeting with Shiki Tohno in Arcueid’s route. Her philosophy on “love” is shared with the Dead Apostle Ancestor present during the French Incident shaped like a grotesque and gigantic spider. This Ancestor claims to have transformed humans into spider-like Ghouls out of “love”, which it defines as the act of watching over others and making them stronger. It is unknown what relation she has to the spider-shaped Ancestor: she and the Ancestor may be the same individual, though she may simply be a vampire sired by it or even one of its human followers.
She appears at the Tohno Mansion soon after Shiki, having been invited by Akiha to serve as a family doctor and advisor, as well as to perform work on the building.
Arach is implied to be the source of the unusual spider-like Ghouls that appear at several points in Arcueid and Ciel’s routes. The same Ghouls appear in Noel’s flashbacks of the French Incident, the creations of a Dead Apostle shaped like a gigantic spider.
When Shiki is left with a leg injury following his battle with Vlov Arkhangel, Arach inspects his body and treats his injury. She mistakes his physical traits as a Nanaya assassin for those of an oni Mixed-Blood, as she is under the impression he is Akiha’s brother by blood rather than adoption.
Several days after this, she examines the chest scars from Shiki’s childhood near-death experience, making cryptic comments in reference to how unusual it was that he survived such an injury. She does not make any further appearances after this, but is presumably still residing at the Tohno Mansion in the end.
Like in Ciel route, Arach attends to Shiki's injuries following his battle with Vlov. She agrees to keep his injuries a secret from Akiha, and inexplicably asks Shiki if there was someone else with him who was also injured, "in a way that can't be seen on the inside". This reminds Shiki of Noel, implying Arach both witnessed the battle and possesses knowledge regarding Noel's past.
She later appears at Shiki's high school, claiming to be administering vaccines for an infection that has been going around. She offers Shiki two vaccines, one which she claims hurts and has little effect, and another which she claims doesn’t hurt and has great effect. If Shiki takes the less painful vaccine, she tells him that “his easy days should soon be over” as he departs. If Shiki takes the more painful vaccine, she becomes excited by his choice and administers the dose in a strange-looking red vial. When the vaccine appears to have no immediate effect, she is disappointed by its lack of results. Two days later, the vaccine leads to Roa being able to take complete control of Shiki’s body.
When Noel is left grievously wounded after a violent clash with Ciel, a "spider" observes her from the shadows. During her desperate prayer to God for strength, Arach suddenly appears, claiming to be able to grant weak people the strength they desire. She tells Noel that Vlov did not turn her into a vampire, and offers to do so herself. She injects Noel with multiple shots of a serum called Idea Replica, which allows Noel to ascend to a higher stage as a Dead Apostle. After six injections, Noel reaches the VI stage.
In Ciel's Normal End, Dr. Arach is shown to be alive and well at the Tohno mansion at the end, with Shiki never having learned of her involvement with the Dead Apostles or Noel's transformation.
Dr Arach is mainly a scientist, but she's also a qualified architect and can work as a surgeon on a professional level despite not having a license. Despite her eccentric behavior and questionable ethical principles, she is a highly skilled medical doctor. Her skills as a doctor and surgeon allow her to provide not only for normal humans, but also half-Oni such as the Tohno family, including those with highly unstable blood like Akiha.
She also has highly in-depth knowledge on Dead Apostles, as she knows precisely what to use for turning someone like Noel into a Dead Apostle despite the fact that she should not have been able to become one in the first place. It is implied that Arach has some relation to the spider-shaped Dead Apostle Ancestor present during the French Incident, as its deformed, spider-like Ghouls known as Krake (クラーケ, Kuraake?) and Metastasis (メタスタシス, Metasutashisu?) appear during both the French Incident and the events in Souya. Following this, she also displays knowledge of Noel's past, making comments regarding her psychology to Shiki after Ciel's battle with Vlov. In addition, when she transforms Noel into a vampire, she is shown summoning white threads like a spider's web, which she uses to bind Noel in place so she can administer her first injection of Idea Replica. Her spying on Noel is represented as Noel being watched by a small spider, but it is unknown if this is a familiar of Arach's, an alternate form she can take, or something else.
Dr. Arach possesses a special drug known as Idea Replica (イデアレプリカ, idearepurika?), that will force the vampiric conversion upon any human that takes it, even if they would not normally be able to develop into a Dead Apostle. Further doses of this drug accelerate the development process to the point where entire stages of growth are skipped completely, but an increased risk of a lethal overdose occurs with each state this bypassed. It is heavily implied that her serum Idea Replica is an imitation of the Dead Apostle Ancestors' Idea Blood as Noel gains Rose Princess Rita Rozay-en's Mystic Eyes of Roses and her other abilities from Arach's procedure, with Rita's Mystic Eyes being stated to be her Idea Blood.
She is also shown earlier in the route using her authority as a doctor in to administer questionable injections disguised as vaccines to the students of Metropolitan Souya High School. While their exact function is unknown, they are contained in vials resembling her Idea Replica, and when given to the Roa-possessed Shiki, allow Roa to fully take over his body, implying they may be a similar serum used to create Dead Apostles.
In Melty Blood: Type Lumina, it is revealed that the Idea Replica-based Dead Apostle form Noel took on is still at a prototype stage. The more Noel keeps relying on the said artificial Dead Apostle powers and takes her victims' bloods, her body will progressively break down and disintegrate, then die in the process.
Creation and Conception
Takashi Takeuchi describes her as “a new part of the Tohno residence who clearly doesn’t belong there”, and “Tsukihime’s greatest source of funny faces”. According to him, drawing her expressions was the most fun he had doing so in his entire life.
Like Goto Saiki, Arach was intended to be a “foreign substance” within the Tohno mansion in order to make it seem more unsettling. She is described by Nasu as fun to watch, but someone no one can understand or wants to understand. She never hated anyone in the story, and in the bad ending on Day 9 of Ciel’s route in which she injects Shiki with her Idea Replica, she simply viewed him as a snack. There was an even more intense version of Mamiko Noto’s performance as her that Nasu feels sad he had to reject.
 Tsukihime Material I - CHARACTER Profile: Dr. Arach, p.46
- Gender: Female
- Age: Who cares?
- Blood type: Not telling you
- Height: 175cm
- Weight: No way I'm measuring that
- Birthday: Who gives a damn?
- Alignment: Chaotic Good (lying through her teeth)
- Likes: Work, sugar, milady Akiha!
- Dislikes: Nothing, nil, nada!
- Natural enemy: If I must pick something, I guess deadlines?
Comment from the illustrator: A lady doctor living in the Tohno residence. An energetic scientist with a habit of skipping work. A new part of the Tohno residence who clearly doesn’t belong there. Tsukihime’s greatest source of funny faces. Maybe the most fun I had drawing expressions in my entire life. I just drew her silhouette the way I wanted it to be, without thinking much about the details.
- 生日: 興味なーい
- 好きな物: お仕事、糖分、当主ちゃん!
- 嫌いな物: ありませーん!
- 天敵: しいて言うなら納期?
COMMENT from STAFF
- Gender: Female
- Tsukihime -A piece of blue glass moon- - Moon Princess route - Day 2: "A Day of Nothing"
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Day 13: "Mirage"
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Day 9: "Bow of the Skies II"
 Tsukihime Material I - CHARACTER Profile: Vlov Arkhangel, p.36-37
- Gender: Male
- Age: Around 400
- Blood type: O
- Height: 188cm
- Weight: 79kg
- Birthday: May 14th
- Alignment: Chaotic Good
- Likes: His wives, bears, his liege
- Dislikes: Snakes, blizzards, floes
- Natural enemy: Chaos, Rose, Modding Maniac
Comment from the illustrator: The frozen Dead Apostle, number 19 of the 27 Ancestors. The character who best symbolizes the differences between A Piece of Blue Glass Moon and the old game. I intended to design him as a decadent, beautiful and pale pure vampire, in direct opposition to the non-standard vampire in the old game.
- 生日: ５月１４日
- 好きな物: 妻、熊、ご当主
- 嫌いな物: 蛇、吹雪、流氷
- 天敵: 混沌、薔薇、改造魔
COMMENT from STAFF
本作『-A piece of blue glass moon-』と旧作との違いを、一番象徴しているキャラクター。旧作で登場した「異端の吸血鬼」と真逆の、退廃的で美しく顔色の悪い「純度の高い吸血鬼」としてのデザインを目指しました。 (武内崇)
- Gender: Male
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Day 5: "Distant Ocean of Frost"
 Tsukihime Material I - GLOSSARY: Hemonomic Principle, p.100
Idea Blood. The quality of the blood that makes the Ancestors what they are, and a foundational commandment etched on their soul.
The Dead Apostles who inherited this blood would always rise up to become Ancestors no matter their rank; Arcueid compared it to a "crown".
On the other hand, a millennium-class foundation is needed for the Hemonomic Principle to function, and even if a Dead Apostle with hundreds of years of activity were to inherit it they would be crushed by that curse. Properly speaking it was not something that Vlov could withstand, but thanks to a technique given to him by a certain Dead Apostle as well as his desire for revenge against that person he managed to retain his sanity until the end.
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Day 9: "Bow of the Skies I"
- Tsukihime -A piece of blue glass moon- - Moon Princess route - Day 6: "Cerulean Stigma"
- Tsukihime -A piece of blue glass moon- - Moon Princess route - Day 9: "Death."
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Day 6: "Vermillion Vestiges I"
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Day 11: "The Day After" - Bad End
- Tsukihime -A piece of blue glass moon- - Rainbow in the Night route - Normal End - Day 15: "Rainbow at Night"
- Tsukihime -A piece of blue glass moon- - Moon Princess route - Day 12: "Night of Ill-Omen"
- Melty Blood: Type Lumina - Dead Apostle Noel's Arcade Route - Ending
- Interview with 4Gamer