You see, before the worlwide release, characters can flinch during battles. For example, if Player 2s bird is down to half of his or her health, Player 1s bird screams in rage while Player 2s bird flinches in fright. Then, the 2 birds continue fighting and receive damage, only Player 2s bird makes more flinching while Player 1s bird attacks without mercy.
As of the worldwide release, BOTH birds scream in rage when one of the bird is down to half health, but I do wish the old flinching feauture (mentioned above) is brought back.
In the mean time, I strongly think that Stella should join Angry Birds Epic, since the Zombie Pig from Epic is in "Fight!".
P.S. I wish playable Piggy Fighters can be easier to collect.