From the back of the box:
Cher Ami spent his youth on the roof of his home caring for the family pigeons. When war broke out he and his brave pigeons got right into the fight. With his amazing pigeon skills Cher is able to deliver messages with the upmost certainty that they will be delivered. When it's time to call in an artillery barrage Cher Ami is your best bet.
No one ever described Elsa Frost as warm hearted. She is more like a machine than a rivet. But this strong unbreakable presence has instilled a loyal fervor in her troops. When Elsa leads her soldiers into battle it is certain that they will be faster, stronger, or more accurate. One thing is for sure, you had better get out of her way!
note: contained in the "second wave" expansion