I think rajveer is responsible 4 navin's death or may be he has killed navin.This is the thing which raj is hiding from naina b'coz if naina knows this,she will start hating raj.I feel these are the reasons why raj is hiding the truth from naina.
Otherwise there is practically no reason why raj should hide the truth from naina.He knows naina has joined the academy to prove that navin was not a traitor so he should tell her everything but may be for this reason he is hiding the truth.
What do u guys feel?Ur comments are welcome.