Maiko was a normal person living with her single mother and sister Akemi in Misaki Town. As she was constantly caring for her sister while their mother was working, they had a very close bond. As boys began to bully Akemi, Maiko began studying karate to keep them at bay. As they grew up, Akemi entered the same high school as Maiko despite being more capable of entering another, so Maiko tried pushing her away emotionally to get her to stand on her own.
Maiko is very similar to her sister in appearance, but has shorter hair. In Dawn, Maiko manifests with Nrvnqsr's trench coat and body composition. Appearing like a "constructed" outline of a woman's body, her body feels elastic to the touch like soft rubber.
While appearing as Shiki, her body has his face with Nrvnqsr's trench coat.
Maiko is normal girl before her death, very close to her sister. She constantly tried to protect her throughout their life.
Upon reemerging as Nrvnqsr, she is extremely mentally unstable, unable and unwilling to remember her death. She frequently sees a crimson moon as her mental state wanes. During her time as Shiki, she thinks and acts like him, inheriting many of his memories. She still is unsure of some things he should know, questioning certain things as she sees them. She later regains her senses, but she is plagued by the hunger of her body.
Maiko is unnamed in Tsukihime, appearing in the "Near Side Routes" as a simple passerby during the fight between Arcueid Brunestud and Nrvnqsr Chaos. Despite Shiki Tohno attempting to warn her to run, she decides to go try to help instead, feeling confident her karate skills will prevail because it's her sister's birthday. She is quickly killed by a tiger sent by Nrvnqsr gouging out her face with its claws. Her last thoughts are that she doesn't want to ruin her favorite set of clothes given to her by her sister for her birthday. Bringing her back for nourishment, the tiger enters Nrvnqsr's body with her corpse, with the sound of the beasts devouring it coming from Nrvnqsr's body.
Maiko manifests from a piece of Nrvnqsr that had been used to heal Shiki's body after the fight with Nrvnqsr. Maiko Yamase initially believes herself to be Shiki, appearing in his room with his appearance. Waking up at midnight, she notices a great urgent feeling for blood. Taking Shiki's knife, she leaves the estate out the window to avoid waking Akiha Tohno, Kohaku, or Hisui. As she lands outside the gate, she feels the hundreds of beasts inhabiting the chaos, which overwhelm her. She regains her senses in a back alley in front of desecrated human corpses and her hands covered in blood. Ciel appears before her shortly after, paying no heed to the bodies.
Ciel asks her, under the pretense that she is Shiki, about Arcueid, stating that she has been sent back to observe the situation with Arcueid because she has yet to leave the town. She believes the bodies must be the work of Arcueid or another Dead Apostle, asking Maiko if she knows Arcueid's location. She is unable to answer, so Ciel leaves to find her. She begins to question if she really is Shiki, quickly losing consciousness again. Awaking in a park, she contemplates the situation and decides to go home to the Tohno mansion, but she is found by Arcueid.
As Arcueid greets her as Shiki, Maiko senses that she is dangerous, and she quickly determines that Maiko is not Shiki. Leaping back from Arcueid, she is filled with fear as Arcueid questions her while emanating blood lust. As Arcueid takes a measured pace to deal with Maiko, she becomes terrified for her life, unconsciously loosing three black dogs from her body to attack Arcueid. She then realizes her entire body is made of such creatures. Though Arcueid easily dispatches them, it causes her to pause, identifying Maiko as Nrnvqsr Chaos.
While Arcueid contemplaties whether Nrvnqsr simply survived the Mystic Eyes of Death Perception or had a spare body, Maiko has no idea about Arcueid's words, beginning to doubt her memories as there is something she cannot remember else she believes she will die. Arcueid says she will be unable to kill Makio, but believes her strength is enough to hold Maiko there until she can bring Shiki to kill her. She attempts to use Marble Phantasm, but becomes shocked when it is ineffective against Maiko. She asks if Maiko has "begun to approach true chaos," while Maiko sends out more beasts and runs for her life.
She wanders until daybreak, unsure of her true identity and realizing she did kill those people in the alley. She sees the state of her body for the first time, acknowledging that she is not human. Walking through the streets looking for a safe place, she finally remembers her own home. As she finds the hidden key and enters the empty house, she suddenly feels the beasts' hunger once again. Desperate not to repeat the previous situation on the family of the house, she rushes to the kitchen and feeds her body raw meat and all the food from the refrigerator. Instinctively drawn towards her own room, she proceeds upstairs after eating.
Upon seeing a picture of herself, she finally remembers how she was killed and regains her self. Akemi then arrives back home, and Maiko is unsure of what to do. Deciding that she needs to see Akemi, her sister is elated to see her, but Maiko does not allow them to embrace out of fear for the beasts. Although Akemi is overjoyed at her sister being alive, Maiko tells her that she is dead and only back for one day. Akemi is skeptical, but Maiko shows Akemi her body. Maiko then thinks the only reason she has come back is to wish her sister the "Happy Birthday" she couldn't before she died. Unable to stay any longer or comfort her sobbing sister, she leaves the house.
Returning to the park where she died, she notices that the beasts aren't acting up and hopes for some way to prevent more victims should the hunger return. Awaiting for those coming to execute her, she wishes she could see her family more, but knows the beasts will act again. Arcueid, Ciel, and a sickly Shiki arrive shortly after, Ciel attempting to attack Maiko with Black Keys. The attack is ineffective, Arcueid stating that neither of them can harm Maiko in her current state. They explain that only Shiki can kill her with his Mystic Eyes, but that he was made extremely weak after she separated from him.
Deeply saddened by the playful banter between the three of them, she realizes her normal life is gone forever. She is unable to exist without killing due to Nrvnqsr's body. Throwing Shiki his knife, she does not understand how he would be able to kill her until he removes his glasses. Upon seeing his Mystic Eyes, the beasts instinctively react to him, but she quells them. She purposely attacks him while holding back, but thinks there is no way his knife can hurt her. She is stabbed a single time, struck with a fatal wound. As she begins to die, the beasts go berserk, but the Ciel and Arcueid easily dispatch them.
Pleased that she is able to die, she is startled upon her sister arriving at the scene. As a tiger bounds towards her, she makes one final push to run towards it and tackle it before it can devour Akemi. Shiki kills it shortly after, and Maiko collapses. As her sister sobs over her, Maiko attempts to put on her best face and smiles. Unable to protect her sister any longer, she tells Akemi to smile for her as she fades away.
Maiko is a blackbelt in karate, believing herself capable of disarming someone with a knife. With the power of Nrvnqsr, she has great physical capabilities, able to leap thirty meters at once.
She has full possession of the characteristics of Nrvnqsr's body, including his Reality Marble Lair of the Beast King. She is initially unable to control the hunger felt by the body, but is later able to barely keep it in check. After a few hours, she has much better control, but she knows that she will be unable to beat the hunger of the beasts.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Yamase Maiko [Person's name], p.190
Maiko Yamase [Person's name]
A young girl who went missing during the course of a certain incident.
She appears in the Kagetsu Tohya side story "Dawn", and has a younger sister named Akemi.
As for who she is and what kind of incident she was dragged into, that's covered in the story.
For now, let's just say it's tying off a loose end from the original Tsukihime.
- Kagetsu Tohya - Dawn