Jasmine 'Jaz' Parks is an agent with the CIA.
At 25 years of age she is already a very experienced and very respected member of the small department in which she is assigned having had quite a few missions, most of them very successful. She has led a team for a while and easily could again or, if needed, she could operate well on her own. It is not through any lack of talent or ability or drive that she is now the "assistant" to a senior agent. In fact, it is because she is so good that she is seconded to him.
Vayl is the name of the primary agent and he is easily the best that the Agency has ever had. During the night, that is. During the day he is particularly vulnerable and that is where Parks comes in. She is his assistant and his backup and, when the sun is shining, his bodyguard.
This is a world in which outré creatures exist. Witches use magic. "Weres" change into animal forms. Ghosts still hang around long after their former bodies are dust. And vampires, like Vayl, stalk the night.
In Vayl's case, he has done so for 300 years and many of those have been in the line of work he still does and still enjoys - being an operative for his country. For at least 50 years his country has been the United States and with his supernatural abilities he is the best the nation had ever had in that capacity. Still, he has his vulnerabilities and of late he has realized that having a human helping him and watching over him during his vulnerable period each day is growing more important. His legend is impressive and so is the list of those who would like to shove a stake through his heart.
That is where someone of Parks' abilities is so vital. She really can, and does, take care of herself and now that she has for the past six months at the start of the recorded adventures been teamed with Vayl, she has on several occasions taken care of him too.
The world in which Parks operates has despots and crooked politicians and gangs and warlords and terrorists and so on. It also has many nasty and scary things that go bump in the night.