[quote=Lord Commander Tritonis]
Could I play a Space Marine??
[/quote]

From what I've read, it depends. From the OP:
"Customizable characters from video games are okay as long as they have some sort of canon personality and are not just heroic mimes. Mass Effect Shepard and Persona Protagonists work out in this regard while characters who solely exist just for gameplay do not."
In other words, a completely nameless and personalityless Player Avatar is not okay, but, say, Master Chief is.