Adjusting a flat character

When I started thinking about my project (an adventure series) I decided on three main characters - A timid hero named Theo, his girlfriend Aria and Mika, the outsider who pushes them out of their status quo and into adventure. However, as I've developed the story I've begun to have problems with the characters. Essentially it's a shonen project and I wanted to play with the archetypes (Theo's the hero who's too timid to act, Mika's the hero who's obssession with her goal often works to her detriment and Aria is the chick with the most common traits of the hero).

As the projects progressed I've realised that Aria hasn't got a real journey in the story. Theo's journey has him slowly growing into a not so typical heroic role. Mika's goal is the backbone of the story and her journey is defined by that. Aria however hasn't progressed much passed being Theo's girlfriend. When I first created the character I wanted her to be the most powerful member of the group and that's a trait I want to keep but I'm having trouble defining her role.

Aria travels alongside Theo because after growing up in an isolated village she has a yearning for adventure. As the story goes on we learn that the village's isolation is related to Aria's birth (they believe that her birth has ensured the village's survival and that if anything were to happen to her the good fortune would be reversed) and agents have been sent from the village to bring her back. At the moment she acts as a kind of middle ground between Theo & Mika, often quelling arguments between the two.

Any advice? I feel like as a character she's going to come off as kind of flat compared to the other two. I'm open to anything at the moment.

1) Have her decide to go back, willingly, either because she wants to change something about the isolation of her village and keep other people from feeling as stifled as her, or because she's had enough adventure to decide that it was exciting, but not something she felt like living for.

2) At the very least, write a few scenes where she goes off to do something without the other two. They don't need to go into the main story, it's just an exercise to see how she develops as a character.

Well, you could do one of the two things I do to flesh out flat characters:

a) look at what I've had them doing and try to explain it from their perspective, based on their personality

b) take a character idea in search of a plot and see if it fits them

If you hadn't started already I would have suggested combining the characters of Mika and Aria. The only advice I can offer at the moment is to have her get captured for the rest of the story so she's simply out of the way. That or kill her.

One way to flesh out a character is to start looking at things from her viewpoint and then ask questions. Does she want to be the savior of her village? Does she want these powers? If so, why? If not, why not? What lies does she tell herself? What truths does she reveal to her companions? What does she expect to gain from her adventures? What is she willing to lose? How will she change, and what will cause those changes? What will she learn or unlearn?

And most importantly, what is her favorite pizza topping!

What? Knowing what kind of pizza your character likes is a great way to build personality.

Thanks a lot for all of the advice, it's come in really handy. I decided to go with the very first suggestion but I'm going to make it last for the majority of the story. I like the idea of the world outside of her village being a lot different than she would've expected and kind of playing with what that does to morale.

Thanks again, you guys are awesome.

: Oh, and don't forget your character's perferred type of sleepwear!

Like a full suit of armor?

You could say that, but someone would most likely take his/her armor off first...unless it's a Clingy Costume.