You've put down on paper the very basics of what your main character will look like, and basically how they will act in given situations. Now comes the hard part- giving your new character a personality. For this you will have to sit down and really think about the people around you; the ones you know well enough to be able to draw a clear picture of them in your head- flaws and all. Think about the way your closest friend thinks, the little quirks that make her unique, the endearing qualities as well as the hurtful ones. Decide what you like about her and what, if she changed, would make her less desirable as a friend. Maybe she's a bit pudgy but sees no reason to change her eating habits making her super fun at a dinner but less fun for clothes shopping. Maybe she's a bit critical when you feel all puffed up but is ALWAYS there when you are ready to cry. Whatever...she's your friend and she's real.
This step should also be well thought out before you ever put this lovely creation into your scene. Write it down and really describe her before you begin to imagine her doing anything. This is sort of like mixing and baking the cake before you add the icing. This way you can gradually add the description of your main character to the story for a more natural and compelling feel. This also leaves room for you to change your mind while writing but be very wary of doing this as you can destroy what makes your character so lifelike if you tinker too much.
Next you might try writing out some dialogue between yourself and your character so that you get a feel for how she might sound, think, and react. Create a little scenelet where you are stuck together for some reason and you have no one to talk to but this new person. Get a feel for how quickly this person begins to trust you, open up to you, and reacts under minor stress. Make this as real as possible...give her time to open up but assume that she isn't going to give you her whole life history in one conversation! She's going to talk about what's most important to her at this particular point in her life.
Now take some time to think about how you would describe this person to a friend later if you had just met them while standing in line at the supermarket? What sticks in your mind about her from just your conversation? This will help you when you go to write fictional accounts because you will be describing a person you met briefly and interacted with. You'll be able to see times where your character is acting out of character as they unfold because you will know that this "feels" wrong. She may do the dumbest things but she WILL do them because it's in her character to do them. She may realize and try to moderate the behavior later but for now she's stuck.
In part 3 we will talk about breathing life into your main character while they interact with the perfect world you have created.