Charlie won't admit she's afraid to die. She doesn't act like she is, because she takes dangerous risks (they are calculated risks, she's not reckless) on the battlefield, especially after she's stronger and more durable because of her cybernetics, and she'll make jokes about how she died once and came back, so what's to stop her from doing it again if someone actually manages to kill her. In reality, dying is something she's terrified of, because when she died over Alchera it was horrible, gave her nightmares she never really stops having, and is not something she wants to repeat. I do think she eventually admits that she is scared of dying to Kaidan though, probably somewhere towards the end of ME3 (I think it might come up in one of their conversations they have when they're back on Earth before the final push to the beam up to the Citadel).