He is a member of the Tohno household living in Misaki Town. He possesses the Mystic Eyes of Death Perception, Mystic Eyes that allow him to see “death.” He is the main protagonist of Tsukihime and the Tsukihime series.
Shiki is in fact not a real member of the Tohno family, is not Akiha’s biological brother, and is not Makihisa’s son. While the Tohno family is a family with demon blood, Shiki’s real surname is Nanaya, a family of powerful demon-slaying assassins. It is revealed in Kagetsu Tohya that he is the son of Kiri Nanaya. The Nanaya clan lived in a secluded mansion in the mountains, but was one night destroyed by the Tohno and Kouma Kishima. Shiki was the only survivor of that attack. In a moment of amused irony, Makihisa spared and adopted the boy because his name was homophonic with the name of Makihisa’s eldest son, Shiki.
Shiki lived in a small Japanese-style building on the grounds of the Tohno’s western-style mansion for a time, learning to accept and be accepted by his new siblings. Although he was lonely and unwilling to leave his room at first, eventually he warmed to the cheerful red-haired girl Hisui who would come every day to ask him to play, and in turn lead his shy and often-nervous sister outside as well. He, his sister, his brother, and the red-haired girl would play together frequently.
Eight years before the start of the game, he was involved in a life-threatening accident, which led to anemic symptoms and left him with a large scar on his chest. Due to these symptoms, he suffers from frequent collapsing and fainting. Eight years before the beginning of the game, everything changed. Although Shiki did not know it, the Tohno family’s demon blood led to inhuman urges in all its members; every descendant eventually became insane from its effects, a process called “inversion impulse”. When this happens, the person loses all ability for conscious thought and becomes a monster with only the mind to kill and consume others. One hot summer day as Shiki played with his brother SHIKI and sister Akiha, SHIKI inverted without warning, and immediately attacked Akiha. Shiki managed to shield Akiha at the cost of his own life; the wound that SHIKI caused by piercing him in the chest became the large scar that Shiki would bear from that day forward.
It is the duty of the Tohno’s family head to police demon activity in the area, and specifically to execute any Tohno who underwent inversion so that he or she cannot do the sort of damage that such inhuman beings inevitably do. Makihisa arrived on the scene immediately and, without a second thought, killed his own son. However, neither Shiki nor SHIKI died that day. Although Shiki did not know it then, his sister Akiha provided him with half of her own lifeforce, keeping him alive. In turn, SHIKI instinctively stole half of Shiki’s borrowed lifeforce to avoid his own death. It is this lack of energy that causes Shiki’s anemic tendencies.
In addition to the physical problems, the accident also caused Shiki’s eyes to develop a peculiar ability, the ability to see lines superimposed on objects and people. If Shiki traces these lines with a sharp tool, the object immediately falls apart along them regardless of the amount of force used. Upon discovery of this ability, Shiki understood how destructive it was and how fragile the world around him could be. Because his eyes would not return to normal, the realization began to gnaw at him.
Fortunately, a few days after his time in the hospital, he ran outside and met Aoko Aozaki. Aoko (whom Shiki decided to call Sensei) comforted him during the next few days. Shiki was eager to impress Aoko, thus he demonstrated the ability of his eyes by destroying a tree. Aoko reprimanded him by slapping him and explained to him that what he had done was unjust. Aoko later provided a pair of magical glasses which, when worn, removed the lines from sight.
When Makihisa realized this convoluted state of affairs, he concocted a ruse to deceive the outside world. As the head of a publicly important family, which held an influential position in business, he could not allow others to know that the eldest son of the family had become inhuman, died, or disappeared. To conceal this, Makihisa locked SHIKI away and hypnotized Shiki into believing that he was the Tohno’s eldest son instead, erasing the details of that day from the boy’s mind. After installing Shiki in SHIKI’s place as eldest son, Makihisa then sent Shiki away to live with the Arima family branch under the pretext that a boy so dreadfully injured, and who could die at any time from a fragile constitution could not be the heir to the Tohno house. Despite the necessity of making Shiki believe in his new identity as a Tohno, however, Makihisa retains an antique knife inscribed with the name of the Nanaya clan, and later bequeathes it to Shiki, although the ambivalent nature of Japanese kanji causes Shiki to misread the inscription as “Nanatsu-Yoru”, or “Seven Nights”.
Because of his poor health, Shiki was removed from the Tohno household by his father, Makihisa Tohno, despite being the eldest son of the family. Instead, he was sent to live with a distant branch of the Tohno family named Arima. He frequented the practice of doctor Sougen Jinan during this time, and once had his first sexual experience with his daughter, Tokie Jinan. At the start of the game, Shiki was called back to the Tohno mansion after eight years of absence due to Makihisa’s death, called back by his younger sister, Akiha Tohno. Although he had not seen his sister in those eight years, she has grown into a proper lady of a high-class family, and after removing all of the family residents and servants of the mansion except for a pair of maids, had chosen to invite him to return.