The Case For Killing The "Blank Slate" Character
It's common for many works of fiction to include a main character who is in essence a blank slate. These characters' tastes, traits, and motivations are for the most part vaguely defined so that audiences can (supposedly) project themselves onto the characters and thus identify themselves with them.
They may like music, but it won't get any more definite than "classical" or "pop" or the music of some artist that almost everyone agrees is or was great. They may enjoy reading or watching movies, but what they read or watch won't be defined beyond a few "classic" titles or genres that the target audience will likely enjoy - if their tastes in fiction are defined at all.
They might also have a few characteristics that have been carefully chosen so as to resonate with the work's target audience. In stories where there are mysteries to be solved, they'll probably show interest in solving them. In superhero stories, they'll likely be interested in fighting for justice and making a difference. In stories aimed at teens and young adults, they'll often feel like they don't fit in and wish for a place to really and truly belong and/or feel like they're meant for something more exciting and meaningful than their current banal existences.
Their personality quirks and idiosyncrasies will be nigh nonexistent, and their flaws will never reflect poorly upon them. They may be clumsy, but they'll never be arrogant or snobby. They may have powers they have a hard time controlling, but they'll never be obnoxiously obstinate. They may get tongue-tied around their crushes, but they'll never be vain.
Now, to get one thing out of the way: there is a place where blank slates do have a place, and that's in reader stories and gamebooks ala the Choose Your Own Adventure series. The protagonists of these stories need to be ambiguous so that readers have room to imagine that they are the characters whom the stories are about. But with any other work of fiction, it's another story - there's no real necessity for blank slate characters.
The trouble with blank slate characters in other works of fiction is that while the other characters in the stories are allowed to be vibrantly alive with a variety of emotions, passions, flaws, and quirks, blank slates are not. As a result, they often end up feeling bland and lifeless by comparison. If the stories progress long enough, their lack of personality and individuality will often end up perceived as a trait in and of itself - rather than being seen as "the ones who could be anyone," they become "the boring ones with no personality."
What they actually do accomplish in the story could just as easily be done by more developed characters, too. For example, blank slates are often used to introduce audience to the settings by putting them in the role of the "new person." In this role, they'll be new students/employees, rookie agents, or people who have just stumbled upon the truth. Thus they're supposed to introduce the audience to the setting - they'll be able to ask questions that the audience might be asking or have things explained to them that the audience needs to know.
Now, there's no denying that this is potentially a very useful role to fill. It doesn't work to simply leave your audience in the dark about what's going on, and exposition via narrative can quickly become tedious. However, once the audience knows the the setting, the "new person" is no longer needed. And thus the blank slate newbies eventually become irrelevant.
Sometimes writers will try to keep their blank slates relevant by giving them mysterious origins to uncover and/or by chucking new powers onto them, but there are a few problems here. First, you can only reveal a mysterious origin once per character, and you can only add so many new powers before you end up making them overpowered. Secondly, even the most exciting origins or powers in the world aren't going to change the fact that they're just plain boring people - only genuine character development can do that. Thirdly, you'll very likely end up with a large portion of your audience wondering why so much fuss has to be made over the most boring characters in the story while far more interesting people end up sidelined.
On the other hand, "new person" characters can be as well-developed as any other character - and so rather than creating characters who are supposed to be the audience, you can just as easily create characters whom the audience can learn along with. Then when the "new person" gimmick is no longer needed, you won't be so likely to end up saddled with superfluous characters.
Another place blank slates are frequently found is in the romance genre. Here, they're often cast in both the roles of the main characters and their love interests. Supposedly, readers can imagine that they are the leads of these stories and that the love interests are their own ideal love interests. However, this strategy can backfire - when your characters have no real personality or interests to speak of, it's fairly well impossible to write a relationship based on anything beyond physical attraction. There will be people who notice that this is all your pair has, and they'll often end up wondering just why these characters actually stay together and/or how long it'll be before the breakup.
There will, of course, be plenty of people who are satisfied with romance stories starring blank slates, and if you wish to cater to this audience, that's your prerogative. However, if you want to write relationships that come off as really and truly solid, blank slates won't cut it - you need to show what makes these people tick besides each other, and what they enjoy doing together beyond basking in each other's presences or making sweet, steamy passion.
If you're not convinced to get rid of your blank slates, here's one final thing to remember: Excitement and enthusiasm can be contagious. Even if your audiences aren't totally into what your characters are into, seeing them get excited over something they're really and truly interested in can make them feel excited, which means that they're more likely to remember your characters as exciting people.
Bottom line: blank slate characters are mostly unnecessary and may even drag your story down. If you really want people to identify with your cast, give your cast a variety of passions and personalities and make them come from a variety of backgrounds and have them face a variety of difficulties. People may not see themselves wholly in one single character, but they will still likely see aspects of themselves in multiple characters - and even if they don't, they're much less likely to find your characters boring, and you're less likely to end up with a deadweight character on your hands.
If you liked this, you might also be interested in:
Reasons Your Character Might Be Boring
Tips & Ideas To Make Better & More Interesting Powers
Building Better Backstories - Tips & Ideas
Basic Tips For Writing Better Ensemble Casts
Are Your Characters In Love Or Just Infatuated?
Couple Development Questions