Requiem

Unlock Requiem as soon as possible. Her Troop skills all increase Troop stats meaning you want to get her to 211, but she can easily be a hero in your lineup.
Her Inner Force skill paralyzes enemies, while her other two skills punish those paralyzed.

At higher levels she can stand toe to toe with others and dish out some massive damage at the same time.

Requiem is top-tier and a must grab as soon as you can get her.