What episode of Code Geass does kallen find out lelouch is zero?

Answer 1

She finds out in the final episode of the first season She is with Suzaku and he shoots zeros helmet off thus letting karin see his face >:}

Answer 2

If you are referring the time when Lelouch went out on public, revealing that he murdered Clovis and also announcing to everyone that he is Zero, that would be Episode 4: His Name is Zero.

Answer 3

it is on episode 25 of the first season, when Suzaku shot Lelouch, his helmet falls and his identity is revealed to Kallen and Suzaku