Souichirou Kuzuki lives at Ryudou Temple with Issei Ryuudou. He is the history teacher at Shirou's school and is Rin's homeroom teacher. Kuzuki comes off as unemotional and highly disciplined and is well respected for his dedication to his job.
Kuzuki is later revealed to be a highly trained assassin, but he retired some time before the events of fifth Holy Grail War. He wields a unique style of martial arts that Shirou describes as snake-like.
Kuzuki discovered Caster fading away near Ryudou Temple and she requested for his help. Kuzuki helped her without question and Caster fell in love with his sincerity. Kuzuki served as her Master in the war but couldn't provide her with a supply of magical energy since he is not a magus. Caster could remain in the world with the contract she formed with Kuzuki but absorbs the life energy from the townsfolk as her source of energy.
Kuzuki presents Caster as his bride-to-be and is allowed to live at Ryudou Temple in the guise of preparing for their wedding ceremony. Caster keeps Kuzuki and the people of Ryudou Temple in the dark about her true nature and activities.
Unlimited Blade Works
At school, Shirou and Saber met Kuzuki at the student council meeting room. Saber noted that Kuzuki has a very controlled breathing and is highly trained in combat. She also deduced that he is not a Master of the war.
Kuzuki reported to Issei that Mitsuzuri had gone missing. She was said to have last been seen with Shinji.
Rin suspected that the Master of Caster is someone from Ryudou Temple. After Shirou investigated Issei, they decided to ambush Kuzuki outside Ryudou Temple. Caster came to his defense against Rin's magic which confirmed Rin's suspicion.
Shirou suspected that Kuzuki was under Caster's control but Kuzuki proved otherwise. Instead, he revealed that he had no knowledge regarding the war and of Caster's activities. Upon learning the truth, he decided to grant Caster freedom to do as she pleased.
Shirou, Rin, and Saber took that as Kuzuki's declaration of war and Saber attacked him. Kuzuki surprised her with his skill in combat and Shirou deduced that he had been strengthened by Caster's magic. Kuzuki managed to defeat Saber and he attacked Rin. Shirou then came to her defense and fought him off. They retreated shortly after.
Later when Shirou, Rin, and Lancer assaulted the church to rescue Saber, Kuzuki dueled against Shirou. Kuzuki defeated Shirou and came to Caster's defense. Archer appeared shortly after and attempted to kill Kuzuki with a shower of swords. Caster took the attack in his place and declared her love for him. Kuzuki then attacked Archer stating that he wanted to continue the war on Caster's behalf. Archer easily killed Kuzuki as he was no longer enhanced by Caster's magic.
Kuzuki was killed by the black shadow when it attacked Ryudou Temple.
Kuzuki is trained in a unique style of martial arts which Shirou describes as snake-like.
Being trained as an assassin, Kuzuki could hide any tell of his attack, making him unreadable and a dangerous opponent in melee.
Kuzuki uses his elbow as a fulcrum to attack his opponent at a right angles from different directions. Kuzuki is also trained to aim at the vulnerable spots of the human body, and his different angles of attack would take his opponent off-guard.
Kuzuki's style revolves around defeating his opponent before s/he has the time to learn the foundation behind it. Once his style is understood, his opponent could predict the possible angle of attack given that s/he could keep up with the speed of his fists and the angle and position of his arms.