Hamster's character, Kristopher, is a prankster who shuns his upper-class heritage. Over time, we came to see him as the main character and a lot of the plot began to revolve around him. This is not something Hamster wanted or intended, but it wasn't until after some time that we realized how bad this was for the roleplay. Hamster felt crushed by the responsibility. She didn't want to make all of the important plot decisions. At the same time, other characters (especially Tiger's) were not getting the "screen-time" that they deserved.
The solution I came up with for this problem was this: for one session, Kristopher would not exist. In-game, he would be sick, but none of the other characters would be allowed to interact with him or even talk about him. This was effective only because by this point all of the players had multiple characters, so Hamster could just play as another one of her characters. If your game comes into a similar situation, but your players each have one character, consider severely injuring the character. Partially incapacitating them by breaking her leg, striking her blind, giving her memory loss, etc. will make her less effective and force the other characters to step into the spotlight.
Don't force any of your characters into a Mary Sue situation (http://www.springhole.net/writing/marysue.htm). No character should be more essential than any other. If you can all admit that the character could die and the story would still be good, then you are still okay.