How do you create an original character? I find many authors create characters that they can relate too, because they better understand them. It's easier to write about something you understand. I find I, usually, write female leads with strong personalities who's lives aren't going well for one reason or another. I do this because I can get that point of view.
But that's just a start. There's backstory required to give reasons for things, and the character has to fit the plot. They have to react reasonably and believably to situations. This takes a lot of careful consideration.
For instance, the last novel I wrote my female character was in her teens (at the start of the story, she grew through her twenties by the end); she was a bit shy and introverted; she liked her own company and didn't mind not having friends - good, because for most of the story people shun her thinking she's weird - but should could also have a strong personality. She wasn't afraid to stand up for her beliefs - important since she found herself often in situations unusual for a young single woman - but she would never go so far as to change the status quo. Basically, she fit the plot of the story well, while still have a definitive backstory before the plot began. She reacted to things as you would expect someone in that situation to react. She also thought and spoke very much like I do. It made her easy to write. I could look at things through her eyes.
I find I don't create characters the same way twice. Sometimes I have an entire plot of a story before I have the lead. Other times, I have a lead and no story for them. Usually it's somewhere in between. You need the character to fit the situation and vice versa. Even more than that, you need a character that can act with other characters. Whether it's against or with, doesn't really matter, but they have to be able to interact. The character I mentioned above, for instance, likes being on her own, but does interact with others. Some of them she doesn't get along with so she goes to others for help and understanding, which progresses the plot.
It's so much harder than that. I'm going to talk a great deal about the character I'm creating for this years NaNo. I'm going to do that because, beyond a name and a few general background characteristics, I have no concept of her. So, you can watch her unfold as we go and hopefully by November she'll be a fully rounded character within the larger plot.