Question: why is mephisto an exorcist?
Answer: After Shiro Fujimoto's death, he is ordered to kill Rin but instead accepts Rin's proposal of becoming an Exorcist and enrolled him into True Cross Academy. Mephisto is a powerful exorcist - he is an Honorary Knight (but does not hold a Meister), and claims that he desires to "bring peace to both Assiah and Gehenna".