Tharja is a good magic user and probably the best user of dark magic that you will have access to in the early game (aside from the avatar, of course). She does not have any way to obtain Galeforce, so she significantly drops off in the late game when compared to powerful child units. If she is your favorite character, do not be discouraged from using her. She can still be powerful enough to hold her own in combat, but she will not be able to be the star of your army and quickly clear maps. Tharja also makes a great pair-up partner with just the skills that she has upon recruitment.