Tsukihime's Shiki is from the Nanayas.
The Nanaya's are a clan of assassin that hunt the nonhumans and only rely on their own (human) abilities.
After entering the Tohno house and barely surviving SHIKI's attack, he develops his Eyes of Death Perception.
Then you have Ryoki Shiki from KnK.
The clan Ryogi is from ,develops two personalities on it's kids (male and female) for special training purposes (to master a variety of skills yada yada).
After car accident and a coma of TWO years, she lost the male personality, and a 3d one (a weird emotionless mix) appeared, along with the Mystic Eyes of Death perception.
What the 3d personality is exactly about and how the ability is related, different story.