Interesting start. While I concentrate on writing poetry I enjoy reading massive amounts of Fiction.
Since the "Orb" is a key item in the story I would always capitalize the "O"
I believe your solution would be to map out a sequence, very basic as you will fill it in as you go. You have four points of interest you need to incorporate in your story so make a list of points on a page for each. No idea what you will actually do but the following is a sample that I hope helps, it will work for short story or long one to just focus your thoughts. As your imagination goes to work you can add, change or ignore any of its points.
It is my impression that if you write a semi complete story for each person, start to semi-finished, then interleave the paragraphs or chapters where they fit best you may have an easier time of this project. Any of our book authors out there may provide better advice for you.
1. Kalina - apprentice, shocked/frightened by Arien, Orin saves, seek solace riding in woods, meets new stranger (call him NEWBOY, who attracts her (later this could prove to be Arien in disguise or not, could be her new hero, surprisingly to her becomes proficient in use of magic even Orin can't do, eventually defeats Arien to much acclaim, kalina gets Newboy or not.
2. Orin's history with the Orb, why he has it, why hiding, what reason Arien has for wanting it.
3. Arien's point of view, why he wants it, what he will do with it, and how he plans to get it .
So that was just a quick idea for you, hope it helps