If you are like me and you are playing this game despite not knowing the language, Margaret is yet another character that needs the proper dialog choice made to get her to join you. Unlike most of the choices you've had to make so far though, the correct option in this case is the second option, not the first.
As for the requirements and the like, a good solution for this is already up on the US version.
Solution for Margaret In Spectral Force 3: Innocent Rage