Mist and Elincia have good things about either of them.
Mist makes the BK boss battle a lot easier, she's Ike's sister, and she's great healer
Elincia is an Est archetype, but her sword level will take sometime. If you use Amiti anyway, then it doesn't matter
NOTE: Est archetypes are characters that come late, and start out with low levels. However, they get insane growth rates.