"Higashi Key" by Leo Annakin Belkin

Higashi Key was called to service because his predecessor Naguri Rin had one too many arguments with Fait and she no longer wanted him as her messenger. The Kirin in my world work directly under Fait. Sometimes they lead glamorous lives and steer world happenings from an obvious position of power. Other times they are very discreet and their actions seem insignificant. They have a will of their own, but are required to suppress it if they want to gain the power that comes with the title. By blood kirin are the specifically engineered children of a dragon and a unicorn. They are also never raised knowing about their heritage or potential. Higashi himself was a child prostitute before Fait finally had need for him. Unlike his predecessors (he is the 5th) Higashi had a great working relationship with Fate and managed to get her to trust him enough to make his own decisions and largely create his own destiny. He is also the first Kirin who had an assistant, Cauley Rin (Naguri Rin's half breed son). Higashi Key is known as the longest serving Kirin of all time.