Shiki Tohno
The protagonist of Tsukihime.

At first glance, he appears to be your average highschooler, but he was trained in anti-demon techniques at an early age, and is pretty physically talented for a human.

At a young age when still known as 'Shiki Nanaya' he had been met with a near death experience at the hands of his adoptive brother 'SHIKI Tohno', and was gifted the “Mystic Eyes of Death Perception,” which allows him to see the death of others. By cutting the “Lines of Death,” which are only visible to Shiki, he can cut nearly anything.

He can also “kill” by striking the “point of death".

A short time after that near death experience and some identity shuffling making him believe he was Shiki Tohno, he met 'Aoko Aozaki' who would change his life forever; Gifting him glasses that hide those dreadful lines of death.

For the comfort of Aoko, he calls only her 'Sensei'.

Skipping over the main premise of Tsukihime because it's really long and convoluted, despite probably being the simplest work by Kinoko Nasu.

Shiki Nanaya
Many years after that near death experience, Shiki had developed a split personality, Nanaya.
Nanaya is more or less Tohno's bloodlust manifested into his own being after some complicated circumstances including White Len. He takes great enjoyment in violence.

Please keep this in mind, Tohno is a normal dude while Nanaya is that same guy just really serious. Both have the same voice.