Which of these characters should I focus on raising?

Are you talking about Cordy and Florence? They're both strong attackers. Cordy is a bit easier to raise. (Because you can exchange her in the Time Shop.)

If you consider 'raising' as 'evolving,' then Cordy and Florence should be prioritized. Cordy is further along, so I think prioritizing Cordy is a good idea. Others include Primavera. Next would be Nina and Mimi. The permanent characters to prioritize for evolution are Cordy, Merlin, Primavera, and Florence. Next in line are Nina, Mimi, Dian, Amleth, and Meltyu.