[@vancexentan] noted, I've made some edits. Tell me what you think?

Also, in regards to what a Miko is; the short answer is that they are Shinto priestesses who (like Jinshu, their male counterparts) maintain shrines, administrate religious ceremonies, and generally are responsible for the pacification of restless spirits. 

A Miko is, in modern times, pretty much a part time job you pick up during high school to make some quick cash. You mostly clean shrines and thank people for donations. It's kind of boring (according to my late wife) but is important because the job of a Miko is primarily to [i]pacify[/i] restless spirits as opposed to vanquishing evil spirits like a lot of western-inspired games tend to put it. 

Throughout history Miko employed archery as a method of self defence and after the introduction of Buddhism and Zen philosophy archery became common practice among Miko as a recreational and meditative activity as well as for defence. Miko were also known for being learned healers and counselors. Basically, most priests and priestesses studied medicine, archery, and oral/written wisdoms in addition to spiritual balance. Shinto Priests/Priestesses were integral parts of their communities and sometimes even took on leadership roles when necessary.

As a job class, I would say that a Miko is a magic user whose greatest strengths are in protection magic and healing magic, and probably subclasses into alchemy and archery. 

I would like to note, however, that Nana doesn't have any archery skills and blessed arrow/ blessed arrow barrage are magic skills with similar function to that of a magic missile but take the form of an arrow as a nod to the close relationship between Miko and archery throughout history.