To the people who gave all the good tips:
First of all thank you for your cooperation.
Second, I have seen all your comments and put a few here that were slightly different form the other comments. 
 

To the readers:
lilyemc  on May 31, 2013 14:32:43 says: x
Okay, okay, okay, I've read bits and pieces of the other responses on here and there is something I have to disagree with. Don't make a character a Mary Sue? Why not? Mary Sues are typical and cliche and boring, sure, but it is still a character type. There's the bad boy, the good guy, the wise guy, the smart girl, the Mary Sue, and the list goes on. So, what's wrong with having a Mary Sue? What's wrong with having that cliche character in your character roster? (Nothing, that's what!)

So your story has more than ten characters, what should you do? Base them off of real people? You could do that. Or, you could simply base them off of the situation they are in. A person is made up by what they experience and how they react to these experiences. Say you have your OC almost get hit by a car being driven by one of the male love interests (let's call him MLI). Does MLI act like a jerk about it and care more about his car or does he genuinely feel sorry for what he's done or does he freak out about going to jail if he had hit the OC? If he's the first, then he's materialistic, most likely grew up with either richer parents who provided him with everything or poorer parents who valued the money they made over the people they interacted with to get it (that's for you to decide). If he's the second, he's the nice guy, the normal guy, who feels bad but does he still drive off with the intent to never see the OC again or does he offer to take her to where she's going or buy her something to compensate for almost taking her life (and does he do this because he thinks a human life can be bought or because he simply doesn't know what else he can do to make up for it because there's simply nothing he can do to compensate for the OC's life almost being taken?).

Then, there's the last one, maybe he has a bad record at school or his university or overall and if he gets one more strike, he's done (maybe one of his parents did something similar and the emotional trauma and scars has left him feeling utterly scared of the idea of ever committing a crime). The situation he is in (almost hitting the OC with his car) and the way he reacts to it shape him in the story and to the readers of the story. So, there's no problem with using a Mary Sue, it's just that it depends on how you use them. How they interact with the story as a whole. How do other characters interact with the story as a whole as well as the situations they are in that make up the story? That's their character. That's what I focus on anyway.

LadyVamp  on Jun 1, 2013 01:58:31 says: x
Most of the time, I just make it up as I go along. But, if I'm having trouble defining a character, I sometimes sit down and create a back story for them. For example: write down where they were born, what their childhood was like, have they had any traumas..., and so on. Then, when you go to write the story you can explain why the character acts the way they do. I hope that helped?
 

dream_keeper88  on Jun 1, 2013 10:47:48 says: x
Well, working with more than three characters is difficult enough for me since I have to give them separate voices. Anyway, as the others suggested, observe people around you and design your characters based on them. 

Here in AFF, most find it easy to write stories with multiple characters, because they write members of a real kpop group. Just watch their interviews and reality show where they appeared. Read magazine interviews. By watching them, you'll have an idea what kind of person the artist is. Their habits, mannerisms, likes, etc. 

Since you are dealing with multiple characters, I also suggest you take time to fill up a character information sheet. There are templates available online or you could make your own, like a personal data sheet. This is for your reference only. Posting it in the foreword is a turn off for me personally. 

I hope that helps.
 

To read all of the answers click here: How can you make characters with different personalities?