How do I gain support in Fire Emblem Path of Radiance and Fire Emblem Radiant Dawn?

I've played both games many times but I still have no idea how to gain support. In FE: PoR my first gameplay I remember getting Rolf and Taruneo to a B support but then in my recent gameplay I only got them to C I dont get it. But in FE: RD I have no idea how to get support everyone stays at C. Some one please help.