Stella is a new, weirdly gendered looking Angry Birds game described by Rovio as this:
Get ready to meet Stella and her friends! Stella may be pink, but she's definitely one of the Angry Birds, a fierce and a feisty character with a great group of friends. Of course no story is complete without some interesting (fr)enemies!
Angry Birds Stella will open a door to a part of the Angry Birds universe you've never seen before! There will be plenty of fun and adventure, but the bunch of passionate pals will also have to deal with some serious issues in order to protect their friendship and the environment they live in. Luckily, Stella and her friends are good at finding creative solutions!
The game is due some time this fall.