No one actually knows yet, but there have been afew suggestions such as: Ellen Page, Kristen Stewart, Haiden Paineter, or that girl from spy kids (Carmen) but it could be someone entirely unknown....
She doesnt die i don't think but if she did it would probably be versing Ari again or one of the flock just like in Percy Jackson except Percy doesnt die but its not going to be till the last book if...